2.3 only reads SD card Emunand and installs .nsp in "Emutendo" folder).
You don't downgrade SX OS, you just replace the boot.dat in the root of the SD card to the version you need. So actually you need to use 2.21 boot.dat again.
Then, if you want to use Emunand on 2.3, delete your Emunand with 2.21, replace boot.dat with 2.3 and create a new Emunand. If you want to use previously installed titles, rename "Nintendo" folder to "Emutendo".
If you are ok with Emunand at 2.21, just stop after replacing the boot.dat.