I think the idea is a good one, but I feel that there are too many dangers associated with it as well. What happens if you're planning an upgrade and before you have a chance to purchase the upgrade, the software expires? You're pretty much S.O.L. Not a situation I'd like to be in...
Here's an even worse scenario : what happens if the company that plans to make an update runs out of money before they can do so, and scraps the project altogether? Then you're really SOL