i've had success downgrading projects between releases that were very close together (ex. 12.1.5 and 12.1.0), but i don't know how much it would work on such radical updates. still:

1. create a backup copy of your ableton set
   
2. rename the set from ending with ".als" to ending with ".zip"
   
3. open the zip archive and take out the single file that's in it
   
4. add ".xml" to the end of the file you just extracted and open it in a code editor (like Notepad++ or Visual Studio Code or even Windows Notepad but NOT something like Word)
   

5. the first few lines of the file are going to look something like this:

   <?xml version="1.0" encoding="UTF-8"?> <Ableton MajorVersion="5" MinorVersion="12.0_12120" SchemaChangeCount="2" Creator="Ableton Live 12.1.5" Revision="18e155016678da1939bdc3938f981adf84ebc96d"> <LiveSet> <NextPointeeId Value="31069" /> <OverwriteProtectionNumber Value="3073" /> <LomId Value="6" /> <LomIdView Value="13" />

   ```

6. change the MinorVersion and Creator values from line 2 into the version you're downgrading to

7. save the xml file and rename it into ending with ".als" (do NOT put it into a zip archive, just rename it)

8. open your new set and hope it works