This page contains the reasoning behind the KirriCorp League's errata on the card Soulswap, shown below in its original form.

Name: Soulswap
Type: Spell
Civilization: Nature
Cost: 3
Rules Text: [Shield trigger]. You may choose a creature in the battle zone and put it into it's owner's mana zone. If you do, choose a non-evolution creature in that player's mana zone that costs the same as or less than the number of cards in that mana zone. That player puts that creature into the battle zone.
Mana Number: 1
Set: Shockwaves of the Shattered Rainbow

As with all KC errata, justification of this argument relies on the semi-rigid card costing system given within Duel Masters. No specific examples of the cost system relate to Soulswap, however, the card allows any effect to fundamentally be used one turn before the player's mana zone is 'ready' to activate it. It also allows the summon of a larger creature for only the cost of Soulswap.

Ability: Unconditional targeted removal of a creature from the battle zone to the mana zone.
Cost: 5.
Reference: Natural Snare (+1 Cost for Shield Trigger)

Ability: 'Opponent's choice' removal of a creature from the battle zone to the mana zone.
Cost: 3.
Reference: Vine Charger (+1 Cost for Charger)

Soulswap's main abilities, known as those used when the caster targets their own creature, are powerful. They range from being able to summon Corile on the third turn following a Poisonous Mushroom summon, to being able to use nearly any battle zone entry effect of one of their own creatures in the mid and late game when the card is triggered, all the way to summoning a multicolored creature for which the caster has no mana that would allow it to be summoned in a normal way. The Soulswap player does pay a price for these abilities. Firstly, they gain no 'new' summon. They must remove one of their previously summoned creatures in order to have a new one. Secondly, the 'cost' of Soulswap itself, not in terms of mana, but as a card.

Despite being extremely powerful, the card can be played around in multiple strange ways, and the effects it has usually approximate to what would happen were the summoned card played instead of the Soulswap. Though it may be able to put a Twin-Cannon Skyterror on the field for only a cost of 3, it also removes at least one creature to do this, and though the card is sent to mana and therefore not lost in the general sense, it cannot be argued that 'it isn't the same as having to summon the Skyterror' since one creature's potential attack, or at least presence, is removed, and the Soulswap is used up.

This approximates to 'You may summon this creature for a cost of 3 if you have 6 or more mana. If you do, put one of your creatures into your mana zone.' Powerful, yes, but it has minimal effect on the timing of most cards, merely their general costing. This extreme variability was not considered sufficient reason to place any errata on Soulswap. Even the ability to summon creatures which do not match one's mana available was not considered reason. (If there is one Twin-Cannon in mana, one could summon another if one drew it rather than Soulswap).

This left the ability of Soulswap to target the opponent's creatures, as its possible 'unfair' aspect. The card has the fundamental ability to convert a formidable creature into a much less dangerous one, which can often then be destroyed in some far more convenient way. Contrasts with other spells that removed creatures quickly showed that this was primarily only useful when the caster had either a way to destroy the smaller creature via spell, or when the creature was a blocker standing in the way of the final attacks that would lead to victory.

In the case of a blocker, the other spells used to remove a blocker standing in the way cost less than Soulswap, so this being an optional use of the card could not be considered unfair, especially since a creature would need to be replaced regardless.

In the case of converting a larger creature into a smaller one that could be destroyed by a spell such as Crimson Hammer, for example, it was noted that, cost wise, this equals out to a Hopeless Vortex, which would have the same effect, changing only where the cards end up, and possibly dodging any 'destroy' triggered effect on the original targeted card. Considering that it would cost the caster two cards from hand rather than one, this was deemed fair enough also.

Finally, the ability to trigger and swap one attacking creature for another was examined. Firstly, the possibility of converting something large enough to get past a blocker, into something too small to do so. Still, this was only about as strong as a Solar Ray in terms of stopping most situations. Similarly for a Spiral Gate, which would also be unconditional removal of even a single creature, even one that had already attacked. Even the conversion of a powerful Evolution creature into something smaller had to be considered fair after much analysis.

This left only the fact that Soulswap induces summoning sickness in the swapped creature. At first this seemed to be even less useful than Spiral Gate or Solar Ray, until further examination was made. Solar Ray does not make the creature any easier to destroy, nor does it remove the effect the creature is providing, if any. Spiral Gate removes the creature altogether, but not only may the creature return, but if it is an Evolution or Speed Attacker, it will return capable of making an attack. If the creature had a continuous effect, the opponent has a single turn to make use of the fact that Spiral Gate has removed this effect.

This caused the reasoning to take a different turn. If we assume that Soulswap is a fair method of weakening an opposing creature, and stopping potential attacks or removing special effects, at a cost of three, the question then becomes, where is the 'cost' of all the other abilities the card holds? It was then considered to change the card so that it would only be able to target a specific player's cards on that player's turn, but that changed the card quite drastically, removing the versatility of weakening effects, blocker removal, and such. Tactically, it made use of the card far less interesting.

Due to this, it was decided to remove the 'stop potential attack' effect of the card. Still, given exactly what it would do whilst performing this effect, it would still have required a cost of four in order to remain a Shield Trigger. The original intent was simply to give the card ' if this card is triggered, the swapped creature enters the battle zone in the same state as the targeted creature' but this caused trouble regarding certain rulings and made the card more powerful in multiple situations.

In order to have the cost of three remain, a 'penalty' was added. Namely, that regardless of what the card targets, the creature that appears enters the battle zone untapped and without summoning sickness. This was considered to be a minimal change to the card, whilst removing the ability to outright stop an attack, yet retaining every other possible effect of Soulswap.

If triggered, it can still be used to swap a large creature for a smaller one that cannot necessarily pass a blocker. It can convert an unblockable creature into something that can be blocked. It can stop attacks altogether possibly if an attacker is swapped for a creature that cannot attack. It can still be used to convert something that has already attacked into something smaller (e.g. a Twin-Cannon Skyterror for a Poisonous Mushroom) or used to remove the special effect of a card that was just summoned (e.g. a Toel, Vizier of Hope for a Ballas, Vizier of Electrons). As penalty, the card put into the battle zone gains the ability to make an attack.

Since one would simply not swap a creature with summoning sickness, or a tapped one, for one that could make an attack, if it would cause defeat, this was deemed sufficiently fair and minimally disruptive to gameplay, whilst evening out the cost:ability ratio of Soulswap.

Result:

Name: Soulswap
Type: Spell
Civilization: Nature
Cost: 3
Rules Text: [Shield trigger]. You may choose a creature in the battle zone and put it into it's owner's mana zone. If you do, choose a non-evolution creature in that player's mana zone that costs the same as or less than the number of cards in that mana zone. That player puts that creature into the battle zone.
(If this Spell was activated as a Shield Trigger, the creature placed into the battle zone does not have summoning sickness.)
Mana Number: 1
Set: Shockwaves of the Shattered Rainbow