A model that's been used for a very, very long time, successfully, is similar to the one you are now using. Sell the current version for a fixed price, and the customer owns it forever. Also sell a support subscription plan, for a fraction of the price of the software, lets say 2.5% of the flat cost, per month. Sometimes, give one year of that with the base purchase. Support contract entitles you to bug fixes and minor feature enhancements (point releases) for free, and free or discounted major upgrades.