has the wrong mount installed, the right mount installed incorrectly, transmission misaligned, the linkage misaligned, the linkage got bent or something. I would not suspect the fluid.
A 2000 Boxster with 55K miles is a good candidate for a Tip fluild/filter service imho but I'm not sure I'd do it right now. I'd be afraid it would cloud the issue of why the transmission's shifting suddenly changed.
I think you need to determine if the motor mount job is the contributing factor the transmission's shifting behavior. Once you get that sorted out, then a fluid/filter change would be a good thing.
Sincerely,
MarcW.