As BOBBLEHAT says, if you go by the rule book then any new clearance form the new controller cancels the old clearance, so technically you do not have to be FL150 at SABRE unless the new controller restates that restriction. However, as many controllers assume that you will still make the restriction, then in your own interest, I suggest that you do it anyway.
There was another thread on this subject where PPRuNe Radar quoted the rule, but I can't find that thread.
Airclues