It depends on who registered the keycodes originally. Sometimes you can download them by doing a 'retrieve' from KRS. Othertimes you can only see what was generated, but not download the file.
What I've done in this situation is generate a try-n-buy keycode against the system ID. Since the keycode files are cumulative in the BCM 50 and BCM 4.0, you'll get the keycode file with all the parameters associated with the system, as well as the new try-n-buy. When the try-n-buy expires, the permanently licensed features continue to work. that might be worthwhile for you to try out, provided you've got KRS access.