I'm going to agree with the notion that it sort of depends on the change.
If a character's abilities should change so as to increase his spell casting power, and if the upgrade is nothing more than increasing the stats of the rote to keep up with the increased spell casting ability, the rote should upgrade pretty much instantly. I mean, you're not changing the tools used, the nature of the spell, or even much about the narrative impact (other than power). I see this sort of thing as a natural change the reflects an increase in power.
On the other end of the spectrum, there's totally changing one's rotes. This type of thing should be pretty infrequent. It should generally only happen when something big happens, something like an Aspect change, especially a High Concept Aspect change.