Adapting your play style and kit is a big part of vanilla, so locking our kits for the entire match would give unconventional kits a big advantage
Let's say for example:
I'm fighting khalih, and he really likes using mace, but the kit that I happen to be using has no snowfall or cobwebs
I'm going to lose that fight every single time, but if I can change my kit to one with cobwebs and slowfall I will have a pretty decent chance of winning
I think it would be best to only let people swap between pre-saved kits, since making kits mid match would take too much time
Maybe 5-10 seconds between rounds to change kits?