Dutch Archaeologists clash about city age...

[Jona Lendering]

Also, I find very unlikely it was just a fashion in naming. This hypothesis doesn't explain why we have the emperors names near the town/tribe names.

But that's something completely else... that's just flattery, and as the Sabora inscription proves, if you renamed a town, that did not change the legal position of the town.

if it was just about ancestral laws and no Roman grant, how do you explain documents like lex Irnitana?

I can't. It may have something to do with the spread of Latin rights, as indicated by the elder Pliny. But there is simply no proof that the Flavian Municipal law has something do with offering a presumed rank of municipium.

However how many inscriptions do we have for status changes in general?

If a city became a colonia, it was celebrated; there are texts about offering this title. So it's odd that there's only one text about the offering of municipium rights - although the Praeneste case proves exactly Millar's point: the city wanted to return to ancestral rights, and did not move "up" from colonia to municipium.

Thanks for the references; will look at it later - it half past two over here...