I've bought them all, after all, at that price, what did I have to loose?
Ian.M: I've not tried to re-protect block 4, but why should it not work, because everything else seems to work perfectly.
The 18Fxx39 seems to be no more than a 18F252 wearing a mask, and for most of my projects, I am not concerned about protecting code.
I realize at this moment, I might have overthought the problem.
I might only have had to bulk erase the 18Fxx39 with the programmer doing a chip erase while set for a18F252.
I haven't tried this, the programmer might prevent a bulk chip erase operation for the entire xx39, or more likely, just allowing the lower 24K area to be valid flash pgm. space, and the compiler would certainly agree to this limitation.
The 18Fxx39 might only need to be bulk erased to accomplish the name change. The induction motor code might be protected from chip erase by rules of use only, that is, the user is told to only to add code without ever doing a total bulk erase, just erasing/burning flash blocks other than block four.