I normally do not change formation during the game unless my opponent does so.
I have been playing around a little this season in the cups and league against easier opponents, tonight I changed formation three times but quality remained more or less the same, somewhere around 46 v 32.
Possession never changed more then 1% which in its self you cannot put down to the formations, as play does fluctuate.
I'm pretty certain now that what you start off with is pretty much what you will have at the end, as long as 11 players stay on the pitch and nothing stupid is done like switching to an illegal formation.

As others have also mentioned, the way the game engine runs anything is possible in the second half.