there is a very high chance for an upgrade. Both species will take a long time (2 years) before coming close to their max sizes.
As for bioload, it should be fine as I do large water changes and from what AQadvisor suggests the current stock for the 75g is 80% ish. In the past according to AQadvisor I have kept my 75g at 130% stock for years, never an issue.
Petricola will max out at 4" and being a 48" tank thats 12 times its body length which is plenty for swimming space. I keep the tank very open to allow for lots of swiming room.
Currently it looks completely empty when you walk by my 75g. People ask is there fish in there..