It's one thing to not be able to abuse the system, but before major pvp begins on the server it should be very important that people can switch between combat schools during combat.
It will otherwise become a one-sided game in case you did not pack the proper moves against the enemy, but even if this is acceptable the worst case scenarios will end up being you getting attacked while having the wrong deck out, and being incapable of fighting properly due to having a deck made for say, melee attacks while you have no melee weapons. Or a deck made for hunting when getting chased down by a player.
Now, the balance issue here relies on being able to switch too quickly; so I suggest there could be a long cooldown (Similar to Quell the Beast from world 7?) while switching, where you will have to move away from the enemy and outspeed them in order to focus your combat/tactics and then be able to fight properly.