This is not about using 20.04 when one runs 24.04, the installer allows for that.

The idea is having updated the phone to the shiny last and best version, say 24.04-1.3, one learns that an essential feature is now not working any more.

How to get back to 24.04-1.1 ? I don't see that in the installer, it installs the latest and greatest.

Now this week-end I learned of a command line tool to manage updating: system-image-cli.

We learned that it's possible to update to 26.04 using:

sudo system-image-cli --switch 26.04-1.x/arm64/android9plus/daily

now having switched to 26.04, if one wants to be back to the tested and true 24.04, it's easy:

sudo system-image-cli  --switch 24.04-1.x/arm64/android9plus/stable

However, this is bringing back the latest 24.04 version, 1.3, not the first version that could be upgraded to the desired value.

When on 24.04-1.3 it's possible to revert to say 24.04-1.1 but the syntax is cheesy;

sudo system-image-cli --build 1 -m 2

it's difficult to get it right, the way to go is to add the -n (--dry-run) switch, that will display what will do the installer without the delay in downloading / rebooting / updating.

Hope this helps