a.net has a Delta widebody tracking thread with some sporadic updates on the 777 retrofit schedule. It looks like the 777-200ERs will be retrofitted before the 777-200LRs, but I don't think there's anything confirming this. The updates are sporadic and spread among lots of other discussion, so the usefulness is (imo) relatively low, but it's the best information I've found so far. According to that thread, N863DA flew to SIN 2/17 and will be there until May for a heavy check and the D1 Suite/PE mod.
We could really use a tracking thread here similar to the ones for the other fleet retrofits, especially now that the first ship is in progress...
Delta Widebody Thread - 2018 - Page 4 - Airliners.net