Interchangeable thumbs are a great idea, but heed this warning: Do not get Switch Grips... At least not the last version. I have had 2 sleeves break after 30 or so games, luckily during practice, but annoying nonetheless. I would recommend a type which actually screws into the ball, making breaking a near impossible feat. I am actually switching back to conventionally drilled thumbs for a while b/c I've been turned off by the bad luck I had with them, although the consistent fit was great.
Also be sure that the pitch is done properly on each ball, or you will end up with the same issue of slightly different 'feel'.