Hey! Chiming in from MAMEdev-land and I gotta say that I love what you've done with the place already. ❤
I also help moderate
https://www.vogons.org (VOGONS forums) and about six months ago we went through a transition from a rather old version of phpBB we last updated about 5-7 years ago to a much newer phpBB with a heavily modded skin or two and many new features. There was some mild user rebellion there too, but in the end the people who don't like the change will simply leave. Hopefully there's enough workarounds to stop that loss tho. In the end the folks at VOGONS have received many compliments as well. It's the same every time you make a big change. Usually eventually the veterans will forget what the old version looked and felt like. You'll be surprised how good you can make it if you stay on top of user feedback for a few months in threads like this. I sorta have a standing offer to help modernize the once popular MAMEWorld Forums, which aren't technically the official MAMEdev forums but are instead just a long-lasting part of the community, but MAMEWorld is on some ancient version of UBB.threads and I can just tell that the whole migration will be terrible.
Viewing vBulletin Transition A (Square Avatars) theme at approximately 80% in Chrome on my old HP W2072a LCD Monitor set to 1600x900 looks freaking great. I prefer it to the other vBulletin skin already. I will try to give the other skins a chance tho. You may be able to gather some stats on which is more popular months after you've made this transition
Let's see... actual feedback:
1. The broken emojis are rather glaring and leap out at me as I try to parse their text. For example, I just saw an older post where someone made a reply and ended it with "
". I KNOW that would have been a little emoji/emoticon. How we worked around this at VOGONS was that we were still on the stock emoji/emoticon set from phpBB, no custom mods or collections, so the forum admin who handled the software upgrade was able to map the old ones to the new ones and handle it...
2. I am currently writing this post in the "Quick Reply" section at the end of the thread, and I thought to maybe highlight a phrase and cut and paste it elsewhere. However, the editor frame did not show the highlighting or color inversion of the highlighted words or anything. Perhaps this is what you mean by a "contrast issue"? (I just noticed this was clearly asked and answered in another thread.)
3. I'm also a little surprised that the frame to the left of the "Quick Reply" shows my avatar but not my username and the rest of my profile information. I guess I get why tho,
4. You're probably gonna catch a lot of flack if thread tags don't return with people both able to create new tags AND see all the old ones. That feature was much loved and used (and abused). I noticed that you did say that it will return later, but I did not find that statement to clearly say that the old tags will be returning and reapplied.
5. Another thing which must return is the karma points system, if possible. I haven't discovered where that is hiding yet. That feature has been a forum cultural touchstone for several years now. It would be difficult to read that thread where people give out points again.
There was a notification on where it was in the announcements section AND it's the top selection in Preferences! 
