What you would need to do is issue an End-of-Life notification for all products beyond a certain version, then offer the ability for customers to purchase an extended service contract (like you do with the Lite version of the products). This is how you take care of the old customers that purchased under the one-time fee plan.

Here at our company, I would totally be willing to buy a DSS License for ~$1,000 USD and an addt'l support contract for like $250-350/year. If you can honestly say this would not result in increased revenue and the chance for explosive growth, I don't get it.

Assuming you get at least 30-40% of your existing user base to purchase the extended support contracts and then 100% of all new customers on the new plan with a supplement support contract you are talking some serious dollar signs here. You establish recurring revenue from a once dead user-base and you establish a new line of revenue for future sales. In 2 years, each customer has already paid for an additional license, thus you can issue free upgrades within the given support contract period. Even if you were to reduce the amount of new sales by a fraction, you are still seeing increased revenue on the front-end to compensate for it.

This is just my opinion and I am speaking for myself when I say I would be willing to pay for support.