Will old edits/sequences (movies) on 5.0 open and work in 5.1 if I install 5.1?

Thanks

yes it will,
but you will not be able to work with older version of FCP after that.this means -if you are working on two different computers, one has 5.0 and another one - 5.1. this will not work both ways.
you cannot go back and forth from one computer to another because version 5.1. project will not be readable by FCP 5.0.
opposite is possible (5.1 CAN read 5.0), and 5.1. version always changes your project from 5.0 to 5.1 and you cannot go back again to 5.0. hope this is helps.
filip

We often open 5.1 projects in 5.0 by using XML export. We chose XML version 1 and it works great !