I agree that all warnings should be enabled by default, even if this means that previously warning free code starts to complain.

I would think of these new warnings as a free bit of peer review, rather than annoying. After all if there is a problem with production code I want it to be mentioned.

If the code was warning free and you don't want new warnings then switch off warnings. You can switch it back on if you make changes in which case you are in a good position to address the new warnings.