By going to your site and authorizing it to access their subscription data with their Google login, like you said in the following sentence. That counts as helping, no?
My question was rhetorical. Users can help, but they shouldn't. This is not their responsibility. This is the equivalent of Netflix interrupting my service because I need to help them to migrate their payment provider and they cannot do it if I don't do some manual process.
How many users do you think will bother? No, they'll just complain at some point in the future, probably long after Google has completely turned down the API and migrations are no longer possible.