the switch from religion A to religion B can have two reasons:
- best bonus in your cities (this means there are more cities with religion B than with religion A, or that a particular city needs the bonus, has B and doesn't have A. Pacifism is the best example where you could choose rel B for just one city, the GP farm)
- relations with AIs (as far as i know, human players don't get the modifiers

) : you need good relation to trade, to bribe for war, to avoid being attacked, ..., and the religious modifier can be the easiest way to get there.
There is no point in switching to the newest religion, as far as i can see.
I can imagine you want your new religion to spread, and when in theocracy you may need to switch for this to happen, but you would lose all the bonus from theo, doing this... Bad move, IMHO.