Something I have been considering for a similar situation is the use of the Alt namespace.

What you could do is release Alt::Foo::v1, and also new Alt::Foo::Bar::v1 etc that depends on Alt::Foo::v1. Downstream Makefile.PLs that need the old compatability depend on the Alt* version, but nothing need change in actual code.

This one-time change by your downstream users is perhaps a middle ground