If the new API is (clearly) better, I'd reverse that and add use Some::Module qw( API2009 ); and have a few wrapper functions that are backward compatible. The docs can then say that this possibility will be removed in version x.yy or from 2020-09-10 forward.

As a side note: there is absolutely no need for a return immediately following a croak. The croak will exit the current scope immediately causing the return to never execute. Code like that is misleading and wrong.