really the only difference is that reservoir size. While the bore certainly could be different between what you have and what you get, that could happen regardless and really is something you can control by ordering a different year or model of master.

So that being said, it should not matter if you go ahead and get a disc master now.

Bore size is a different discussion, and I found I had to decrease the bore size to get good braking power. early small diameter boosters were dual diaphram for disc cars vs single for drum cars.
they generally don't provide enough boost.