>How about giving owners of VFP7 a 100% credit (of what they spent on VFP7) toward the cost of an MSDN subscription? That way, the VFP developer would feel like they made their loyalty to VFP clear.

That pretty much exists in a way right now. If you own VFP 6.0 or VFP 7.0, you can buy MSDN Subscription (any level, pro/ent/univ) at the upgrade price rather than the full subscription price which is a savings at or more than what a VFP upgrade would cost.