I wondered if the opposite could be made into a modification?
To, Update the xf_user table when a user's upgrade has expired
Possible options (usage scenario):
• Blank the user's Custom Title field (empty string)
• Blank the user's Signature field (so again, to enter an empty string)
• Set their mood_id to x (ideal when XenMoods is being used as an upgrade)
Ideally the plug-in would be set to a cron. <- Bit obvious but I thought I'd mention it