12au7 versus 12au7A

i was just wondering if these tubes are interchangible.if not then what is the difference? thanks.
The letter indicates a "later and modified version which can be substituted for any previous version but not vice-versa" according to my GE tube manual. It also says the letter does not convey any information about the nature of the modification.
Both tubes are electrically identical. The "A" is a slightly more rugged construction which results in lower output microphonics (and the "WA" is a military spec version which is made for high vibration environments). I cannot see why they would not be interchangeable.
thanx gang for the info.