I came to suggest exactly this, though I'm not sure I've ever seen it in the wild.
Set a flat minimum amount you need to collect for the software and allow your customer to either pay that out in subscription payments (and then continue paying if she wants to keep receiving updates) or, if she decides to cancel her subscription early, allow her to pay any remaining difference between the sum of her subscription payments and your flat price.
Set a flat minimum amount you need to collect for the software and allow your customer to either pay that out in subscription payments (and then continue paying if she wants to keep receiving updates) or, if she decides to cancel her subscription early, allow her to pay any remaining difference between the sum of her subscription payments and your flat price.