I'm a little unclear on the new wildcard rules.When exactly can you switch abilities.Under the new rules could I for example:
Move a wildcard through a wall using the Serpent society TA and stop next to a friendly character,end of action.Then the friendly character attacks with a ranged attack.Could I at that time switch to Hydra for the plus one attack?

Yes, you could do that. The only restriction for WCs is that you can only use one TA per action. In your example, the ranged attack of the friendly figure is a seperate action from the move action, so you can freely change TAs.

A couple examples of some things you cannot do would be --

1) Copying the Serpent Society to phase through a wall while ALSO copying the Green Lantern ability to carry 8 non-fliers through the wall with you, or

2) Copying the Hydra ability so your adjacent ranged attacker gets a +1 to hit while ALSO copying the SHIELD ability so he will gain +1 to his damage value too.