I read this thread and have been trying to rack my brains as to where I had seen the rules about this written. I, too, thought that a new descent clearance cancels any previous level restriction. I even poured over our MATS pt 2 for almost 30 seconds to see if anything was in there! And then it came to me... in a round about sort of way and thanks to GATCO's impressive organ.
For any TC ATCOs (or others) who are interested in the debate started by this thread, when you have a spare minute in the ready room pick up a copy of GATCO's Transmit (Winter 2001) and turn to the bottom of page 17 and read 'Safety Matters'.
For those of you who don't have access, or who wouldn't be seen dead reading Transmit
, here is what it says (edited for relevance):
From NATS ATM Policy & Performance
A clearance issued with a restriction to be "level by" becomes null and void when a futher climb/descent clearance is issued before the original has been fully complied with. Where, according to Standing Agrrements, ATSUs are able to transfer aircraft in receipt of such conditional clearances, the receiving sector should be particularly cautious in issuing further climb/descent clearances. Further, controllers are reminded that any re-clearance instructions issued should also include the original climb/descent restiction where this is still required.
And that appears to be about the long and short of it.