Way too early to be talking about this.
Its too early to make a decision...but is it really too early for a discussion? Not like there's much else to talk about right now.
In any case I think that the final decision on this rule is going to have to boil down to just how extreme the differences are between the styles. At the moment though: I think that SFIV may provide a decent template to work from in this regard. In SFIV there is a character lock...however the winner *IS* allowed to change their Ultra after the opponent changes their character, but *before* they select their own ultra.
Hypothetical example: Reo's cryomancer subzero defeats Pig of the huts Quan Chi. Pig goes to character select and selects Scorpion. Reo now has the option to stick with cryomancer or change to grandmaster, but *must* make a decision before Pig selects his own style of Scorpion. Make sense?
Addendum: Not sure if this matches the rules for SFIV or not, but I agree with 300official's post also that if the losing player only chooses to change their style the winning player should probably be locked also.
This ruleset makes sense to me because IMO it still allows for soft counterpicking and provides options to the losing player, but will also
hopefully help to cut down on the amount of hardcounterpicking that can really take the fun out of a game.