That doesn't sound right. The OP purchased a Lufthansa flight operated under Lufthansa AOC and it was changed to operating under Eurowings AOC.

Regulation EC 2111/2005 requires LH to clearly state the identity of the operating carrier, hence we can conclude it's quite important information. In this case it changed from Lufthansa to Eurowings, a totally different operating company. There is no need for some special Regulation for rebooking because simple basic contract law deals with it. This is like cancelled flights under EC261 Article 8c, flying later at pax's choosing, which is governed by statute of limitations that varies considerably across EU MS meaning you can't present some 10 year old ticket at the FRA ticket counter and demand immediate transportation.

There is a simple change in the contract. Would you say the same thing if it was LH operated by FR?

Over the summer, there was a separate issue of the state of registration when EW flights were swapped out for leased Lithuanian carriers. Buy German, fly Lithuanian. These are all material changes in my book. In the US, under DOT rules, when the carrier changes there is a full refund. They have that rule because something changed.

When BA lease aircraft, they offer pax re-routing on another BA-operated flight. Recently this occurred with whole load of BA flights which were moved to AY and IB. I don't imagine BArb does this out of the goodness of her heart, do you?