Yes, you want to restore configurations onto the same version, and then upgrade, especially between versions 5 and 6 because there were many changes made in the way things are configured. (e.g. simple queues) - the upgrade process should be able to translate these older configurations into the current version, but if you try to import the configuration of the older version directly onto a device running v6.32.4, the newer firmware won’t accept many of the older commands.
If you want to upgrade the hardware to a 951, then the best thing would be to first upgrade the 751 to 6.32.4 and then export the resulting configuration from the 751 and import it onto the new 951 running v6.32.4
Basically - change versions or change hardware - never change both at the same time.