the file that gets modified for the forever root is under /system, which gets wiped when you flash something other than the stock rom. Once you have put a custom rom on your phone the change is gone. If you made a backup of the stock rom, and flash back to that before you do an OTA, you should still have the forever root on the backup (b/c you are restoring the modified /system files)
not sure if that made any sense or not
I see your saying if theOTA comes out and you saved stock with forever root and go back to it should get OTA and still have root.