Sorry, morning coffee didn't kick in yet I guess. ;-) My actual wish is to make all types heap allocated and eliminate the statically allocated ones. So, Py_TPFLAGS_HEAPTYPE would be set on all types in that world. That is a gigantic task, affecting near every Python extension type. Too huge for even a nutty person like me to imagine doing in the near term. So, sorry for potentially derailing discussion here.
I agree with comments made by Stefan Behnel and Petr Viktorin. There is a small risk to cause problems (i.e. serious memory leaks in a previously working program). However, as Petr says, the extension in that case is broken and it is not hard to fix. Eddie has provided examples for what changes are needed.
I think if we properly communicate the change then it is okay to merge the PR.