Here are the few points I think you should keep in mind
- how are you migrating from SAAS to API access business. Is it like one click & everything switch from SAAS to API access or it's chunk wise migration. I think, better approach should inform your clients first about this new process/method & allow them how they want to migrate or even they want to migrate or not?
- Need to think about API scope, security, authentication etc
- API speed & robustness
- API pricing
- API Versioning
- Data Encryption
- API rate limiting and throttling
- API Input Validation and Sanitization
- Caching
- Load balancing
- API Documentation
- Logging & analytics
- Terms of Service and Privacy Policy
- Scalability