I'm in exactly the same situation (I was also pretty surprised to find the flight was now going to Dusseldorf!). My flight has been booked via Egencia, for work reasons; they have told me that
"through the CAA, the EC 261 regulation was amended to classify the COVID-19 situation as "extraordinary circumstances," allowing airlines the ability to implement a voucher refund only policy. Sadly, many of the low cost airlines have chosen to go this route. "
which as far as I can see is nonsense.
Anyway, does having booked via Egencia make any difference in terms of doing a chargeback?