the moment you start pulling up your subfloor you are removing a structural element of your home. Non-load bearing and also loadbearing walls may be plated on top of your subfloor.
First answer, it depends (can a first timer diy'er do this themselves). Some areas that require building permits and code inspections for work within the home even certain projects done by homeowner, do require a building permit for this project.
Second answer, I don't recommend your ripping out/replacing subfloor yourself without guidance/advice from a bona-fide expert who has examined your situation. and if doing I'd go one area at a time, remove one sheet, replace a sheet, move to adjacent area, remove/replace.
This much I know for sure, if you have wall framing on top of the subfloor you mustn't cut it up to that point and just replace/abut the subfloor to this area.
The sub floor acts like a web between those joists distributing sheer forces, torque, twist, load, and the like. Being an important structural aspect of your home its on the same level as like a header, a load bearing wall, etc.