This sounds like terribly boring developer work to do and requires no creativity at all. Is that what's missing when comparing to other types of developer work

This sounds like terribly boring developer work to do and requires no creativity at all. Is that what's missing when comparing to other types of developer work that might keep you more engaged and focused? I'm not suggesting to push back about why it needs porting, I assume that bridge has been crossed already. The other thought is that maybe it was poorly written, having multiple layers of abstraction, etc. that are unnecessary? Most API's are CRUD operations on data or calling deeper internal things, like payment processing, in which case you are calling other API's essentially. I would suggest writing external tests that hit the existing API and in small chunks port features over and incrementally get those tests to pass calling NewAPI. Internal/unit test coverage can help to ensure each module or logical grouping is working the same as the prior work too.