At work I have my workstation set up with a mouse on each side of the keyboard. I'm not aware of any way to swap the buttons on just one mouse through software, but I have never been completely satisfied with the software approach. Instead, I hardwire the buttons to be swapped.
Now I don't even know which mouse I use most often. I catch myself using either. I think I prefer the one on the left because I don't have to move over the numeric keypad to get to it, but more often it has to do with whichever way I am leaning in my chair.
The big advantage is when someone else needs to sit down at my workstation to do something, there is a normal right-handed mouse for them on the right. The only downside is that I always get that question about why I have two mice.
If the Program specifies a version number of this License which applies to it and "any later version"...
As you say, you have to look at LICENSE.txt (I assume; I never actually did) to see that the license on Linux is GPL v2 only.