ukcdmx
Regulation 261/2004 Article 15:
Exclusion of waiver
1. Obligations vis-à-vis passengers pursuant to this Regulation may not be limited or waived, notably by a derogation or restrictive clause in the contract of carriage.
2. If, nevertheless, such a derogation or restrictive clause is applied in respect of a passenger, or if the passenger is not correctly informed of his rights and for that reason has accepted compensation which is inferior to that provided for in this Regulation, the passenger shall still be entitled to take the necessary proceedings before the competent courts or bodies in order to obtain additional compensation.
Regulation 261 provides compensation of 75% of the fare for downgrading. I doubt you would have to go to the courts to have them pay up, they know they will lose.