One quick note: Something I did was I made sure to not let my Mac go to sleep as the downloading process was happening. I tried that the first time and as a result I had to redownload Big Sur.

But by not letting it sleep the second time and seeing it all the way through, I was able to get it downloaded and from there, install.

So far everything seems to be working fine except for the messages app. That isn’t loading. I sent a report to Apple to let them know. Other than that, everything is working. I’ll let you all know what other issues I come across (if any).

I tried messages one more time before I shut out for the night and the app is now working fine. So, in case any of you who upgrade notice the messages app not responding at first, just give it a little bit of time and you should be good.

I downloaded and installed Big Sur on my 2013 MacPro trash can Saturday Nov 14, evening . It is about a 12.8 Gb download, and took about 45 mintures during the dinner hour. It then took about ~45 minutes to install, rebooting several times. The first attempt at installation locked up on a restart step - so I had to finally turn the machine off, and restart it manually, but after that everything seemed to flow properly.

All the native Apple apps seem to work, iMessage particularly works just fine.

I haven't tried Adobe's software yet, as I don't use that on this older device any longer.

Maybe Safari seems a bit snappier.

I did make certain I had two seperate external back up copies of my boot drive before begnning. Never replace an operating system without a reliable 100% funtional back up copy of your existing boot drive. You might find you need it - I didn't this time, but I have needed my back up drive more than once over the years with an OS upgrade.

edit -> I have subsequently added Big Sur to my just recently repaired ( by Apple ) 2019 13" MacBook Air, and it downloaded and installed smoothly and without issues.