Is that a single-line command? It doesn't make any sense, since it's trying to run both the Go and Set commands on a single line.
There was a bug in older versions where it would mis-parse some invalid commands and allow them to run (sometimes with unexpected results), which is probably why it "worked" before. But the error here is the command is simply wrong.
Try Set SOURCE=Toggle by itself if the aim is to switch which side is active.
Or Go TABMOVE=active by itself if the aim is to move the active tab to the other side.
The command in my other post would only work in contexts where you can enter a multi-line command. To use the same thing on that mouse action, you'd need to create a User Command with the two lines inside it, then run that from the mouse action.
Shouldn't be needed if Go TABMOVE=active works on its own.
There is no real "policy". Download links in release notes all point to the same place, if you check the URLs. (Beta versions point to a different place, and we also have installers online for people using older versions, e.g. Opus 11.)
If you want an older Opus 12 installer, we can put them online for you, but it's better to fix whatever is wrong than stay on an out of date version forever and stop updating. You can also keep the previous installer yourself, of course, which is good practice in general with all software. (Well, except software with "installers" that don't include the program and download it from a website every time you run them, but we don't do or like that.)