ASFKAP is right. If you do nothing (i.e. no written rejection of the change), you will have deemed to accept the new conditions in a subsequent tribunal. The length of time under which you 'accept' it by inaction is a bit of a grey area, but typically you can argue under the length of time you need to give notice (for flightdeck this would be 4 months). However, to be on the safe side, I would not leave it more than 1 month due to interpretation of contract law.