You may want to consider one-time (full price) and optional annual renewal fees (a fraction of the full price, depending on the value/upgrades delivered in the new version). It's then up to the customer to pay an annual fee to get updates or stay on the old version (which may be a pain to support...).
Basically, what you mentioned—' I don't want to pay a full price' and 'try the full functionality out without paying the full price'—are the primary drivers behind customers' preference for a subscription-based model.
Basically, what you mentioned—' I don't want to pay a full price' and 'try the full functionality out without paying the full price'—are the primary drivers behind customers' preference for a subscription-based model.