Question

Upgraded to new version from Studio One Artist 2 but all the old mixes (vocals) sound lessened when opened on Studio One 3 pro.

Answer 1

In 3.1 there where some changes to the auto gain of the Compressor and Channel Strip and also some changes to the response of the Limiter and Tricomp which could both affect pre 3.1 songs. So if one of these Inserts are used in your song that could be causing the difference in sound.