If a room type is sold out when a guest is viewing the upgrade opportunities, the upsells page will display the available room types at the top of the page. The "sold out" room types are presented at the bottom of the page:
The image will appear greyed out with the "sold out" logo across the top. Rather than the "request upgrade" button, there will be an option for the guest to be notified if the room becomes available.
When a guest clicks on the "notify me" button, a request is sent to the request center. The request will appear in the same request group as other upsell requests.
The note states that the request was received in the sold out section of the page. If the upgrade becomes available, the hotelier can grant the request as normal.
If the upgrade is not available, simply deny the upgrade and reply to the guest as usual.