Maybe I’m special (and it was 6-10 years ago), but after becoming proficient and pretty deep in osx/ios development I took an android project and it wasn’t obvious AT ALL. If it were a new job and not a project pushed to me at the same position I had, pretty sure it would be difficult to keep it.
Of course that depends on average expectations. Some people may learn and do and relax and still outperform if these are low.