Ohhhhhhhhhhhhh. Now that does make a difference. I did a quick and nothing of any magnitude came back.
I wish I could help you on this one as I've been in your shoes many times, myself - people and boss breathing down your neck for "instant" results but you can't fix something when you don't know how\why it's broken. You try your darndest but nada. At this point, I'd recommend sticking with what's been working thus far - roll them back to 5.0 and keep plugging along. I'm not saying to give up on the migration. Just take a step back, gather your wits and your forces, then re-attack when you've regrouped. You can always try the migration thing in smaller bites, and a little at a time, so you're not in this pickle, you find yourself in, again.
The thing is, most of these people who are breathing down your neck only know that they push a button and things work. They don't understand that you build the button(s) that they push. Without you, they're at a standstill. But that's a concept far beyond their comprehension. Sound familiar?
Hang in there, Brother!