ReadyNAS Pro 6

The steps are

(Optionally) make sure the latest BIOS is installed before you do the conversion.  To do this you install  http://www.readynas.com/download/addons/x86/4.2/BIOS_Update_Package_0.5-x86.bin as an add-on and reboot the NAS.  Do this after you backup the NAS.  This isn't necessary, but if you so want to update the bios it is easiest to do it prior to converion

Basic instructions for the conversion itself are

1. BACKUP, BACKUP, BACKUP
2. Upload PREPR4TOR6_0.1-x86.bin as an add-on using the ReadyNAS web gui, but do not reboot afterwards (avoids the need to do manual factory resets)
3. Upload R4toR6_6.9.5.bin using the ReadyNAS web gui firmware update
4. After you upload the addon+firmware and reboot, it will update the firmware and start a factory default.
5. Go through the setup process on the converted NAS 
6. Update the NAS to the current OS 6 (check for updates will work).
7. Restore files from the backup.

Netgear won't provide paid support on a converted NAS, so that is one consideration.  There is a small risk that the process could fail, and if that were to happen it might not be possible to get the NAS running again.

If you do decide to convert the Pro 6, then you should also consider increasing the RAM.  Shipping OS-6 NAS have at least 2 GB of RAM, your Pro 6 only has 1 GB.  It will work, but it would be sensible to match the resources in the currently shipping NAS.  The upgrade is inexpensive, and is simple to do.