Personally, I think character swapping between matches (and even mid-game) should not be allowed. I think that, like GW1, the build you select when you go into a tPvP chain of matches should be the build you stick with until the end.
Why?
Because it forces players to make trade-offs. Each map is different, and different builds vary in strength and effectiveness on each map. By forcing players to choose a set build for all 3 maps, it makes people have to think deeper about their strategy, their composition and their individual capabilities.
It also creates greater build variety, rather than the typical roles you would see in, say, Khylo, where you usually have the same type of Guardian bunker mid. It would also prevent “cheese” builds, like a heavy multi-bunker build, which may not be as viable on the first 2 maps.
Overall, I think it will create greater depth, variety and strategy to make it so you cannot switch.