This feature is available in the latest version of Upsales Subscriptions. Contact email@example.com to get upgraded for free.
When you win a opportunity and update an existing subscription, a prorated order is required to be created. This means that the change in the subscription must be positive (an upsell).
If you wish to make changes beyond the upsell that result in a negative amount, you must first save the subscription with the upsell and then open the subscription which allows you to reduce the subscription value as desired.