It seems that changing campaign budgets has a much lower rate limit than updating other options: http://developers.facebridge.net/bugs/285486761555036?browse=search_50da9ab10555f6203542865
Even changing the budget three times in quick succession doesn't seem to work. Is this budget-specific rate limit publicized anywhere?
The exact numbers aren't published, but FB did release new API Rate Limiting documentation a few weeks ago.
You can check it out at https://developers.facebook.com/docs/reference/ads-api/api-rate-limiting/#adcampaignlimit.
Daily and lifetime budgets do indeed have their own specific rate limiting (again, without exact numbers).
