I can't find any information on this at all. If anywhere, it would most likely be in the newer (late 20th century) excavation reports, which I also can't find. I would classify this question firmly in the "hard to find" category. Where did you find a reference to the amphitheater being incorporated into fortifications? The Princeton Enc. of Classical Sites does not mention that (see here), though that publication is now pretty dated and also errs on the side of brevity for each entry. It seems clear from tourist pics (I've never been to Rimini, myself) that the substructures still exist in some form, though I gather that most of what was left was destroyed in World War 2, which is often the case.

In general, when ancient monuments were incorporated into fortifications, it meant that their superstructures were used as fortifications themselves. In the case of an amphitheater, it would mean that the outer curve of the structure would become part of the city wall, with all of its entrances, exits, colonnades, etc being bricked up and reinforced. This would probably necessitate that the seating on that outer side be removed or altered for purpose of strengthening the structure. On the other hand, a Hadrianic construction with a rubble concrete core was pretty darn strong already, but not very high. Imagine a single "storey" of height, probably extended with towers or another outer arcade. In any case, the arena itself could have theoretically continued to function, albeit with less seating on the outer hemisphere. I don't have any specific information about this particular case, though. I don't see anything in the limited info available about a 3rd-century fortification. I do see that a wall was built or rebuilt in the Sullan period, but that's 350 years earlier.