There is something wrong with what they are saying. Pretty much everything since 2000 or so (I think that it happened in 99) uses regulators that are smaller in diameter than they used to be. I have an ACI setup, bottle is from Charleton, that was made in 4/99 that has the larger diameter opening. I also have a Charleton Technologies from 08/99 with a Smart Parts reg on it, that is the smaller, now standard, opening. And, just to add to it, I have a Nitro Duck/Paintball Mania tank with a born date of 04/98 that also uses the smaller opening.
I would ask the shop to show you what they mean. Because it should be the other way around, as in the bottle opening is too large for the reg.