If I understand correctly, you’re making some backward-incompatible API changes with the v2 release. So in that case, whether or not your consumers are using Go modules, you must change the import path.
If you do not change the import path, your consumers will get the latest versions with the incompatible changes, whether they want them or not, potentially causing confusion or breakage.
If you do change the import paths for v2, then you are doing the right thing by your users whether or not they use Go modules.
Apologies if I’m missing something here, but as a Go modules user this is my understanding.