OK, in the aftermath of this thread, I emailed Delta and requested original routing credit, as suggested above.

I got a reply apologizing for the delays. I can't quite tell if they're saying they will offer me the original credit or not.

But they said they will give me 10,000 miles because of the inconvenience I experienced.

So I logged on to delta.com and see that they've given me 10,000 miles (non MQM of course).

But the MQMs I got for the trip are still the same, only ATL-LHR instead of ATL-ARN. So I'm missing out on about 400 MQMs, plus the Double MQM promo or about 800 MQMs.

Hmm, I don't know if I should press on. The 10k miles are nice but the MQMs ...

Well if I get the Double MQMs for my trips in May and June, I will be Gold again.

But those 800 MQMs might make the difference on whether I shoot for Platinum this year.