It is hard to be entirely certain but it would appear not.
We have no hard proof of any value changes.
We know that there were many symbols
, but none prior to the M appear universal enough to indicate a change in value.
We know that the size changed several times, but that does not indicate a change in value - in fact it was often done to maintain value during times of inflation.
We know that the M symbol was almost universal and makes little sense as a series name indicator (like A, B and
might) and therefore might indicate value.
We know that there was rampant inflation around this time.
We know Bagnall and Bransbourg date a 10-fold increase in intrinsic value to around this time.
So therefore, to me, it is most likely that there was a change in value around this time and the 6th (M) series is the best candidate, but that it was the only change since 348. Before the 6th series inflation was "addressed" by decreasing module.
I know some authors have postulated a complex series of increasing intrinsic values through marks like A, B,
, E, S and M, but I see no proof. Such a system would be chaotic enough even if every coin were marked with its value, but given the use of such letters was sporadic I don't see how such complexity would be possible.