That was expected, downgrade doesn't work by default.
If you want to downgrade, you'll have to use that disk linked before. That disk contains a Rigol signature hidden with the partition table. It will allow to load (downgrade to) any version, and it will also unlock a few service menus (only when the Rigol special disk is plugged, but don't mess with those service menus).
Prepare an USB pen-drive with the .zip disk image in that link, then copy on it the FW version you want to put into your oscilloscope. Follow the instructions at the end of that link, and it will downgrade.
I've went back and forth between versions a few times back then, and it didn't brick my scope, and It didn't lost the unlocked features. It may reset however the hours-on/power-on counters. In different FW versions these counters are at different addresses, so write those down on a piece of paper if you need them.