1] I should remove the old and add the new mouse when the power is off, right?

Yes, and No! Happy

The OS doesn't really care whether two mouses are connected. But to disconnect the PS/2 mouse the answer is Yes. Unlike USB, anything to be connected/disconnected to a PS/2 port should only be done when the computer is shutdown.

But, connect the USB mouse before shutting down otherwise you may not be able to navigate around Startup without a mouse. It can be done using the keyboard, but not many use that nowadays.

2] Depends upon the mouse. If you connect the USB mouse before shutting down to disconnect the PS/2 mouse, then the question doesn't arise. In any case, you can still use TAB and Enter to select icons in the Welcome Screen. Test it if you wish whilst you still have the PS/2 mouse connected.

3] Windows should recognise the new hardware when you first connect it. I am not sure, but there may be some USB mice that come with a CD with software. If yours doesn't, then just plug it in and Windows will recognise it.

4] Compatibility.

I doubt that will be an issue.

Hope that helps.

Keep that PS/2 mouse handy. USB mouses do fail and sometimes USB ports fail, so having a PS/2 mouse around is always very useful.