Like most "A" tubes they are substitutable otherwise they would have gotten a new number. In many cases the A version has had it's filament warm-up time adjusted to a standard for series filament radio and TV. This makes no matter to us in audio. The 12AX7 is an exception in that they shortened the plate a few mm in the A version but again they are interchangeable.
FYI the largest change I am familiar with is the lineage of the 6L6 where the style went from metal (no letter) to glass (G), then GA, GB all of which had the same characteristics. In the GC they thickened the plate material, made it bigger and boosted the dissipation rating from 19 to 30 watts. However the lower dissipation is design center and the higher one design max which obscures the real amount of increased dissipation.
