We need to untangle what you wrote. Are you saying that you have FreeBSD release 7.x? In that case, you are many years out of date, and you are not going to get any useful suggestions here, other than to upgrade (preferable reinstall). Not because of your mouse, but because that version is simply too old to be safe or supportable.
So let's assume you're running a modern version of FreeBSD. You say you are using command line, no GUI. In that case, what do you do with a mouse? I know that one can select / copy / paste on the console using a mouse, but that seems an unusual usage pattern. I'm not even sure what the 2nd or 3rd button would do in that case.
And to answer your question: No, you don't have to reinstall to switch to another mouse. You don't even have to reboot. If the mouse is supported, you just have to configure the moused program correctly. Do "man moused", and read about it, in particular the "-3" option.