If it helps any, I have done the following flight swaps with ZED tickets:

I had ZM for SFO-CDG on AF.
It was full.
I used my ZM on VS for SFO-LHR.

I had ZM for FCO-ORD on AZ.
Flight cancelled.
I used my ZM for FCO-JFK on AA.

On both, there was no problems in getting it done, not even a second look from the Agents.

I have used ZED tickets since the mid 1990s and it is such a blessing for us!
It basically eliminates the need for many back up tickets. I know some of you still get back ups but I do not. One ZED is all I need!