My understanding is the easy caliber conversions is limited to the mk XIX. Those have a larger frame than the earlier iterations to accommodate the .50 AE. Some of those ship as a package deal, the .50 and a .44 upper. I read you can put on a .357 upper too but it requires a new bolt.
My only experience with these is the mk i .357 my dad has. So everything else is just what I’ve read.
I did find a forum post talking about conversion kits for the old ones from Magnum Research. Apparently they were selling them for a while. They seem to be rare, expensive, and require fitting. The consensus was it’s far easier to simply buy another mk i or vii in the caliber you want.
