My assumption is that the milestones are a fixed thing and, once set and told to the players, cannot be moved or altered; same as any other story or setting element in the game. (I view levels as actual things in the setting rather than as a metagame construct - you know what level you are in-character simply by counting how many times you've trained up)
The problem with this, of course, is that in a non-linear game the players - once they know what-where the next milestone is - are likely to beeline to it for metagame reasons even if it isn't what their characters would normally do.
If however the milestones are unknown to the players and thus can be tweaked or moved after being set then it's not really milestone at all, it's just pure DM fiat.