From CAP493
When an amendment is made to a clearance the new clearance shall be read in fullto 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.
sec 1 chap 4 page 4, para 7