Keeping the old server running during the change is such an easy step to miss. I’d add one more thing to that checklist: save the A, NS and CNAME records before migrating, so you have a before-and-after comparison.

I work on whoisfreaks.com, which offers live and historical DNS lookups. The history can help recover older captured records if nobody saved a copy. It complements the resolver checks you’ve shown when you’re trying to piece together what changed.