Gary,
First, you are right, this really should be a new thread, since someone else needing this info might not look here - however ....
You will have no problems using VFP 6.0 and 7.0 on the same system (unless you choose to chasnge the default location and install it in the same directory!). I've been developing both types of projects on the same system for about a year and a half now, with no conflicts.
If you "migrate" a VFP 6.0 project to VFP 7.0, there are only a couple potential problems. One VFP 7.0 tightened up the syntax checking in places so you might get errors where none existed before. (e.g. Declarations didn't require commas between multiple entries before but now do. So LOCAL a b c - was fine before but 7.0 requires LOCAL a, b, c .) Also, since there are a number of new built-in commands, they may conflict with your own functions (especially if you require different parameters).
The internal structure of the menu file is different so one you open a 6.0 menu in 7.0, it will be converted and won't work back in 6.0.
Last (?) if you choose to implement any of the new database events, then the dBC will be changed so VFP 6.0 won't be able to use it.
In reality it's pretty transparent, and in fact in the VFP 7.0 development environment you can run (almost) any VFP 6.0 .APP or .EXE without even recompiling!
Rick