SirGunslinger wrote:they can both be 40mm, although oldmarines have the option to be 28mm. As long as you don't mind your guys being a little easier to see. Alternately you could convert Mk IV Space Marines, they can be 28mm and they have primaris-style helmets (more accurately primaris have mk IV style helmets) and pretty cool armor.
They're both 32mm. Old Marines can be 25mm.
Only new Repentia and some
AoS models are/were ever on on 28mm bases, and Terminators and Primaris characters are on 40mm bases, but Tactical Marines, Intercessors, and etc. are all currently on 32mm bases.
As for the conversion, go ahead. The parts are cross compatible, though I imagine it might be an expensive endeavor.