I think the scenario where this is clawed back is ridiculously far fetched, and nobody has reported it happening to my knowledge. It would take a pretty serious level of audit to catch too--you got the correct number of MQMs for the class of ticket you FLEW. It just happens that you flew a higher class than you purchased because of a Delta agent reissuing your SDC in a higher fare class.

This has happened to me probably 1 out of every 10 flights, resulting in a good 5-10k extra MQMs per year (usually a paid GAP SDC to an F/J).