Hi Eichoernchen, now that I've sobered up a bit, I'll re-enter the debate...I think Notrab and Unclued have made an argument against leaving behind A,C,E,G : though to my mind Bovril would be completely erased and therefore not a contradiction to the preamble, which states that all 40 entries should be altered. The first across clue (Ace) would be unaltered and therefore a contradiction.
In terms of judging such an entry, perhaps the key word of the instruction is the word 'Erase'. For this to be carried out, there must have been something previously entered. After erasing, presumably traces remain and so the entry may be distinguished from a blank grid.
Apologies to Cockie if my earlier contributions were more than usually brusque !