just theorising here (don't worry, I'm not going to actually try this).
Is it possible to replace the internals of a 3k max-flo with 4k5 internals, to make it a 4k5 reg, or, is it as I suspect, that the body of a 3k is not rated for 4k5, and if filled with 4k5 psi, will send shrapnel everywhere?