ANSA
I think you're introducing grey where there isn't any.
If you revise a routeing profile (either by heading or new waypoints), then that doesn't automatically amend the aircraft's vertical profile and stated restrictions (although see the SID example below), nor does it amend any imposed speed. The only problem it might introduce is if the aircraft is no longer routeing via a waypoint where you have specified a level restriction since the pilot is no longer going there. You can set up a new restriction either as an 'abeam' or a distance from a waypoint in that case if you still need it.
If you revise a speed profile, then it doesn't change anything in respect of a previously issued routeing or level. Why would it ??
If you revise a vertical profile, then it doesn't change the aircraft's cleared route or speed. Again, why would it ??
As
controllerzhu confirms, the procedure is used and, I personally think, works pretty well in the busiest aviation nation on earth, and I also believe it works fine in the UK, which has it's busy bits of sky as well.
From a recent UK CAA Ops Notice to pilots:
Amendment to Clearances
Adhering to the clearance given is clearly important at all times however there are some aspects of ATC clearances that raise questions in pilots’ minds about what they are or are not cleared to do. Two examples of such questions are:
• An aircraft departs on a SID which involves a step climb but is then put on a radar heading. Do the step climbs still apply?
• An aircraft is given a clearance, 'descend FL150, cross XYZ FL180 or below' and then receives another clearance of 'descend FL130'. Does the 'cross XYZ FL180 or below' restriction still apply?
When an amendment is made to a clearance, the new clearance must be given in full to the pilot by the air traffic controller, and this new clearance automatically cancels any previous clearance.
Thus, when an air traffic controller issues a clearance, which amends the route or vertical profile of an aircraft on a SID, e.g. 'climb FL 120', this automatically cancels the vertical profile of the SID. If the profile contains a restriction which provides vertical separation from conflicting traffic on another SID, air traffic controllers must reiterate the restriction, e.g. 'climb FL120, cross XYZ 5000 feet or above'. Similarly, when air traffic controllers issue instructions which amend the SID route, they are to confirm the level profile to be followed e.g. 'fly heading 095, climb FL 80' or 'route direct to ABC, stop climb at altitude 5000 feet'.
Similarly, if the original clearance included a restriction, e.g. ‘cross XYZ FL180 or below' then the issue of a revised clearance automatically cancels the earlier restriction, unless it is reiterated with the revised clearance.