Hi
This is something that comes up with great regularity on PPRUNE.
The legal answer is that any ATC clearance cancels all previous clearances.
WE can debate this again! what the statement means is that if I, as an ATCO wants you to adhere to STAR restrictions, and have given you a heading or a different routing, I have to reiterate the level by restriction. E.G. if you are routing direct to LAM, ABBOT etc if I want you to be level or lower at a certain point I have to say descend FLxxx be FLxxx or below abeam yyy
Some people believe that in a busy environment that is too many words...
I will stand back now