Switching govs is good. even if it means leaving the united planets.
they voted for imperial as standard gov just before i finished researching star fed. so i left u.p. a few turns later and doubled my income from 5000 to 10000 bc with no effect on approval (>95%) also i lost all my traderoutes (< 1000bc).
another bonus was, that i don't have to pay the 20% warfare tax anymore.

i can't see any change in my relations either. so i wonder why one should even be in the u.p.? (except for the trade of course)
i'm currently playing a huge galaxy on tough and a very peacefull game. (for now that is, as conquest is my only victory conditon)