The level by restriction must be reiterated when the clearance is amended.
However the restriction is there to keep you out of someone else's airspace and it would be helpful if you abided by the restriction anyway. If it's quiet and you're unsure, ask.
Some of my colleagues won't give direct routings/continuous descent because of the increase in R/T loading from having to include the level restriction each time.
This has been discussed ad nauseum in the ATC forum and can be found by a simple search.
Sorry I don't do clear cut answers
Here's some links-
first one
http://www.pprune.org/forums/showthr...light=level+by