XenForo does not allow users to cancel their own account upgrades. Account upgrades are canceled ether when they expire or when administrator cancels it.
Some forums might want to allow users to cancel their own account upgrades. For example, if there are several account upgrades that do the same (such as monthly subscription and annual subscription) and there is a limited time offer available for annual subscription, users might want to cancel their current subscription to take advantage of that special offer.
This add-on allows users to cancel their user upgrades, allowing them to take advantage of such special offers without bothering administrator. Administrator can select which account upgrades can be canceled. By default none of account upgrades can be canceled, to enable it edit account upgrade in admin control panel and check "can unsubscribe" box.
Add-on also adds unavailable (only ones that can be purchased if user cancels one of his active account upgrades) account upgrades to list of upgrades and shows which active account upgrade prevents user from subscribing to it.
This add-on is available on GitHub.