AFAIK, firmware downgrades are not possible when using original Rigol firmware.
You must use the hacked .GEL attached by TV84 in order to downgrade from your current version, then run the 'cengen' on a Windows machine. Once the instrument is unlocked (the *IDN? name and the model reported by your AWG will become DG4202), the instrument will remain as DG4202 for any further firmware updates. Then, update the instrument to the latest firmware from Rigol.
Read a few posts before and after the TV84 post linked above, for the exact instructions. The hacked .GEL is inside that 'DG4000 ... .zip' file attached by TV84 at the end in that post.