The only thing that will not work with both versions at the same time is file extension associations, eg a double clicked pjx file will trigger VFP9 to start after installing VFP9.
This is of course just a little bit of trouble, as you can always start VFP6 first and then MODIFY PROJ or USE some DBF or whatever.
Just be prepared some things you begin working on in VFP9 get upgraded in a way they no longer open in VFP6. Eg the anchor property is added to forms and controls and VFP6 can't cope with it, especially once you begin to set these properties in VFP9. And it'll not be handled as any unknown user defined property in VFP6, there are some incompatibilities.
To be able to continue to work on VFP6 projects with VFP6 always work on copies of these projects and all their files with VFP9, then you're on the safe side.
Others might tell you different here, but you can't go wrong separating your VFP6 and 9 versions of things for the same reasons you do backups to be able to go back to some earlier state.
Bye, Olaf.