Between rounds would be nice, but not during a match. If you could do this during a match then you would have people that had maxed all 4 classes with diff chars having an advantage over players with only one or two characters.
It is kind of lame having to choose between staying in a good lobby and swapping chars, but thats why my characters have 2 specializations.
My advice to you would be to either create a few well rounded characters, or to learn how to adapt your playstyle to whats required in each match. Its great to be able to stick with the class you want for an entire match having all the best skills but sometimes you have to switch to a class you dont have points in just to complete objectives.
These issues can be solved by finding a group of friends to roll with, or by just taking it for granted that sometimes you won't have the best loadout selected for your character. (Once again learning the intricacies of your main class will allow you to be dominate most situations)