Database did not upgrade

[DrByte]

Multiple issues that I will resolve somehow now that I've given up on going live for the weekend.

I didn't say this before, but I'm especially puzzled now since you've mentioned it: You're attempting to do way too much all at once. DNS propagation is never instantaneous (although there are advanced ways to minimize risks of downtime, unless you had the forethought to adjust TTLs a few days ahead and reset them after migration) so you should never attempt a move and upgrade at the same time.
It would have been much better to simply keep the old site live until your new site was ENTIRELY ready and tested. Then once you were ready to "go live", you could THEN down-for-maintenance, copy the database, import it, upgrade it, apply any necessary adjustments for mods and customizations, do some last minute testing and then flip the DNS over.
All explained at http://www.zen-cart.com/entry.php?3-...d-of-upgrading

Moving AND making big changes (including upgrading) AT THE SAME TIME is ... well ... something I'd only tackle if it was an emergency. But you just now said you've been planning it for a couple months, so rather than hold my tongue I figured it best to share my recommendation based on much experience.

OK...one question: I admit I skipped a step in above instructions, namely: "Grab the "Fix_Cache_Key" utility to reset your Admin->Sessions->Session Directory key so that it uses the proper folder path for your new server" as I did not understand it's use. Could that have been in issue in all this?

Not likely. Usually the symptoms from that are inability to log in.