https://www.lufthansaexperts.com/sha...ile_143775.pdf
In case of a flight cancellation: If the original booking class is not available for the new travel date, you may currentlyrebook in the lowest available booking class within the same compartment/cabin (original class of travel). This appliesonlyto rebooking to Lufthansa Group flight numbers(OS/LH/LX/SN/EW/EN)
Read § 4.1.2 on page 8 which should apply in your case.