I’m sure they’ll be happy to sell you a business class upgrade. For free? 1% chance.

Assuming there is actually no longer premium economy class being sold on those flights, your options are (1) demand a refund and book another airline, (2) fly in economy and claim/fight for the EC261 downgrade compensation (75% of the fare, prorated for the distance of the segments impacted, but as mentioned upthread I’m not sure if it applies to the return direction).