VFP9 has more error checking built in, so it is a big chance that you were sitting on a bomb when you used VFP9.
First make a backup, just in case...

lnValidate=Set('Tablevalidate')
Set Tablevalidate to 0
Use yourProject.pjx exclusively
Append blank
Delete
Pack
Set Tablevalidate to lnValidate

>Hi!
>I'm having a problem while trying to upgrade from vfp8 to vfp9. When I rebuild the project the message "Memo file MYPROJ.PJT is missing or is invalid." ???? In V8 everithing is fine!
>Any sugestions would be great.
>
>Thank's in advance,
>Nuno Pedro