So tried to do this on recent flights, once they read up on it (I mentioned G class too), they seemed not to have any issues with it, however, never any G fares available.

Since I wasn't successful at using the GUC on my VS flights, I've run into a bunch of agents having issues with upgrading on KLM, stating that there is "Z" fares available for KLM marketed and operated flights but not DL marketed/KLM operated flights. They half that are talking about this are saying that the only way to upgrade these require a full Y fare.

UGH!

My boyfriend calls this a illusory benefit, a benefit that looks like a benefit but doesn't actually confer a benefit in the end.