You could have a look through
MATS Part 1 Section 1 Chapter 4 Page 4, but be warned that it is a 2.1Mb file. The relevant paragraph says
6 Amendments to Clearances
6.1 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
FL 150 or below'then the issue of a revised clearance automatically cancels the
earlier restriction,unless it is reiterated with the revised clearance.