Feel your pain.
I had the same issue when I migrated hosting providers for one of my hosting servers
I use several providers and the hardware was getting old but the price was not viable for an upgrade with that provider ,
that provider was bought by a LARGE corporation which do what they do best for a 14 year client... you become a number that can "please hold" and never get to talk with the actual tech people anymore... because of policy?
long story short some clients had A records set...
I called them on the phone asking to take action, e-mailed, manually placed notices on the invoices for affected clients..
I kept the server online for an extra month just to make sure everybody was happy with the migration.. no "missing emails" or "database entry" during the DNS switch.. , that extra month of
both servers online passed
all of the sudden many tickets started flowing in... "why is my site down" "server issue" "problem with the server" ... and even one chargeback?! ( calm down i won that claim heh).
and don't get me started on those who never EVER upgrade their open source scripts (after notices of-course), guess that's a rant for another day ..
Yeah.. sucks ,but it's part of doing business.
@Peter Laws
May you succeed with all your endeavors
שֶׁתַּצְלִיחַ בְּכָל מַעֲשֶׂה יָדֶיךָ