Rewriting API calls is a week of work. Whether the whole migration is a week or a quarter depends on three things people rarely check until they are committed.
1. Do you store cards for repeat or one-click payments?
2. If so, are they network tokens under your own Token Requestor ID?
3. Where do provider webhooks get handled?
4. Does your database store the provider's own status values?
5. How many providers are you integrated with today?
Indicative. Based on the three things that do not migrate: tokens, webhook semantics and schema vocabulary.