Firmware Upgrade

I think I understand firmware upgrade of the rns-e but just wanted to confirm.
If I want to go BACK a level (say from 330 to 110), so I can use a 110 nav disk, i burn a CD/dvd with the 110 software on it but in the index.txt file change 110 to something higher than 330 (say 340).
The rns-e will then load the 110 firmware thinking it is higher than 330. I should then be able to use the nav disk that the 110 firmware was on, even though the RNS-E says it is at 340.

Have I got this right?
Why do I want to do this? Because I had a copy of a nav disk that had 330 on it and my system upgraded, but as the nav disk is a copy, the map data is not readable for some reason. The stealership version available is at 110 and the 330 software will not read them.

Let me know if I've got something wrong.
By the way, is there anyway for the version number to be reset to 110 so future firmware upgrades don't cause me a problem?

Regards,
BH