It won't change their BPOS to special teams as that is the last grouping checked for BPOS increase. I think defense is first, then offense, then special teams, (though defense and offense could be switched, I'm not sure).

I think many use RB's as special teamers, since RB's aren't really useful at higher levels. As to what type of effect on the players, there's lots of speculation, but I don't know of anything proven. It won't actually affect energy level, but it's speculated that playing multiple positions will have a similar affect on players as having low energy. Maybe somebody else knows something for sure.