I successfully rebooked the canceled flight EDI-TLL-EDI to a 6x more expensive flight FRA-LPA-FRA on a different date by contacting the Ryanair chat support (the return flight, albeit not canceled yet, was rebooked as well). I had to wait about 10 minutes for a human agent on chat, the agent was very helpful and made no trouble. So I can confirm this trick does actually work.

Sadly, I forgot to ask the agent if the rebooking itself had to be done before the original flight date (their regular FAQ on rebooking says so, but maybe there are special rules for Covid-related cancellations).