Since they were originally built for a TV series rather than a movie, they were designed to be permanent, not struck and reassembled like movie sets. They were revamped and modified for each consecutive series, but the underlying superstructure of the sets on Paramount's Stage 9 remained standing from 1978 to 2001 -- a record for the longest continuously standing sets in Hollywood history.
Would the modifications to the TNG sets (I'm specifically thinking of the battle bridge/TOS bridge here - the modifications to the other sets were fairly trivial, limited to Okudagram changes from what I remember) have been designed to be struck, so they could be kept from V to VI? Or was the bridge in VI a rebuild from scratch?
The bridge was rebuilt from the ground up for TFF because it had been modified to death for TNG. Herman Zimmerman designed an entirely new bridge, only the Turbolift alcoves remained from the previous TMP-TVH set.
I really wish they hadn't modified it so extensively on TNG, it was my favourite bridge of any series or film.