Here you go, Mats pt 1, section 1 chpt 4 para 7.1
"When an amendment is made to a clearance the new clearance shall be read in full to the pilot and shall automatically cancel any previous clearance. Controllers must be aware, therefore, that if the original clearance included a restriction, e.g. “cross ABC FL150 or below” then the issue of a revised clearance automatically cancels the earlier restriction, unless it is reiterated with the revised clearance."
Changing frequency doesnt constitute a clearance, so until the next controller gives you a level, you have to stick with the restriction. Personally, its a pain in the a$$ and clogs up rt to have to restate a lot of the restrictions when giving a new level after transfer...