When a Director's Cut version for Star Trek : The Motion Picture was being made, they needed to redo a few shots of the USS Enterprise in flight and when she was inside V'ger. To use back the studio model would have been a lot of work as the Studio Model was repainted over and now had the insignia NCC 1701A. And it would also defeat the purpose of coming out with a Re-mastered version since the difference in details would become very evident.
The Special Screening of the Director's Cut Edition
Some snap shots from Digital Bits of the studio model displayed during the Special Screening of the Director's Cut Star Trek The Motion Picture 0n 11 Jan 2001.