The not an answer flag enters the 10k review queue, where it is then reviewed by 10k users who can either raise an invalid flag, or choose to raise another flag.
After just checking, it seems as though the reviewer is presented with the updated state; rather than the initial state seen upon flagging. Thus, if an edit is made, your flag may not reflect the state presented to the reviewer, as they are seeing the updated/edited state.
I went through over 70+ not an answer flags in order to find one that had been updated since the time it was originally flagged, therefore it doesn't seem to be a very common occurrence.
Here is one of the examples I found:
This answer was flagged at
2013-11-26 14:56:04Z, and later updated at
The updated state of the answer is presented to the reviewer.