Change of basis for rent calculations

We have just received a letter from our Management company PV asking us to sign to agree to basing our rent payments on the L'indice de reference des loyers instead of L'Insee which is based on construction and property costs. The reasoning seems to be (if I have understood the French correctly) that it will suit us all better. As a result our rent will be revised with effect from the next payment. (We are paid annually). Obviously they do not say the rent will decrease as a result but we suspect this may be the case. We thought we had a fixed minimum rent and are concerned we will lose this by signing now. Even if we do not sign, they will apply the changes! The letter was not sent by recommended post! So they do not know we have received it or not. Has anyone else had this letter? Any advice? I have looked at French forums and they sspamest sending a solicitor's letter to refuse the proposal.