If I assigned an ATA to Adam Warlock and he switched to Magus, would Magus be able use the ATA? I'm unsure because the rulebook only mentions meeting the prerequisite (the keyword) when you first assign the ATA, not while using it, such as a feat.

ADDITIONAL TEAM ABILITIES
Additional Team Ability (ATA) cards can be added
to your force if the characters on your team meet
the prerequisite listed on the card. Each ATA card
indicates the cost per character that must be
added to your force in order to use it in the game;
all characters that meet the prerequisites must
be assigned the ATA and your force’s Build Total
is affected accordingly. ATA cards provide either
different or additional team abilities to the qualifying
characters on your force. These team abilities are
possessed by the qualifying characters and can
be used by wild cards (unless the ATA specifies
otherwise). A force may only include one ATA card.

Based on my experience, I would presume the answer is probably no. By the same token that a prerequisite power being countered temporarily takes away access to a feat, AE'ing to another figure who does not share the required keyword would mean that figure would lose access to the ATA.

This is my understanding anyway, I could be wrong.

If Magus doesnt have the required keyword then he cant use the ATA. It would only work when Adam Warlock is in play.

In this case, this ^^^^.

It only works when the required keyword is present.

Yes, Magus would still have the ATA attached to him, but as has already been pointed out, he doesn't have the prerequisite keyword to USE it.

Quick! Switch BACK! "Who AM I fighting, anyway?!?!!"