Which Gerber to Replace my Crane Rexmont That Was Installed in 1999 (or so)?

[Sennal99]

I am about to begin a full second-floor bath remodel in a 1965 house. My current toilet is a Crane Rexmont installed against am interior 2x6 wall, and it has a *1999* date stamped the tank. My understanding is the toilet is a commercial model that was used in a residential application. My question, then, is two-fold. First, if the carrier in the wall can be reused I'll go with a Gerber model. But, if so, do I have to go with a Gerber commerical model (this is more expensive) or can I go with the Maxwell, i.e. a residential model? Second, if, for some reason, the carrier has to be replaced, what is the *potential* cost differential (including parts/labor) for : A/ buying a new carrier to support the Gerber residential toilet; B/ buying a new in-wall carrier/tank so I can go with a more modern tankless toilet? Also, I'm in Alaska, so am trying to figure out my options/possibilities *before* the work begins next week (to account for shipping up here) since, as I've been told, we won't really know what or options are until the toilet is removed and the wall is opened up. My bolt patterns match the Maxwell, but the Crane is hung less than 4" above the floor -- it's more like 3.5".