Upvote:3
Your right to be re-routed under "comparable transport conditions" stems from EC Regulation 261/2004 Article 8 (1), b or c.
Article 8
Right to reimburs*m*nt or re-routing
- Where reference is made to this Article, passengers shall be offered the choice between:
(a) [...]
(b) re-routing, under comparable transport conditions, to their final destination at the earliest opportunity; or
(c) re-routing, under comparable transport conditions, to their final destination at a later date at the passenger's convenience, subject to availability of seats.
http://eur-lex.europa.eu/legal-content/EN/TXT/?uri=celex:32004R0261
However the meaning of the words "comparable transport conditions" is not clarified in the Regulation and, to my knowledge, has never been defined by a court of record invited to adjudicate on it. It's a bit hard to prove a negative, so if anyone else can suggest a relevant case I'm happy to amend this answer. I just did a text search on the ECJ case law on 261/2004.
I can suggest you keep pushing the airline but until a court rules otherwise the reroute via Paris is good enough for their position.