You can disable iMessage at any point and it works. New texts from iMessage users get sent over SMS instead. You don't have to do it in advance—I just did so out of paranoia, and I don't think other people need to do the same.

(Of course, one side effect is that you can't use iMessage any more and iMessage-specific features are unavailable to you, e.g., group texts might get weird. I don't know of any platform that has the property that you can quit it and still use its features.)