Today I upgraded from v1.29.7 to v2.0.0-rc.20. TL;DR: it went super smoothly.
My Syncthing environment has three Devices, I’ll call them HighSpecWin11, LowSpecWin11, and MidRangeMac. All three Devices have SSD drives exclusively. I have just over 600 GiB / 100K files being synced between the two Win11 devices in nine Folders, with under 100 MiB / 20 files on MidRangeMac syncing with both Win11 Devices in two Folders.
I upgraded each Device by configuring “Automatic upgrades” to “Stable releases and release candidates”. This upgraded the Device to v1.30.0-rc.1, and from there I clicked the button to upgrade to v2.0.0-rc.20 . I’m sure I could have upgraded to 2.0.0-rc.20 directly but when I did one via automatic upgrade and it was easy, I did the others that way.
MidRangeMac was upgraded first, this morning, and it created the new v2 database in single-digit seconds. The old database directory is 1.4MB, the new one is 10.3 MB.
This went so smoothly and easily that it gave me comfort for the two Win11 machines. With every Folder showing “Up to Date” I backed up all of them to offline media. At about 6 hours to copy the data over USB-C, this was by far what took the longest.
After that, I upgraded LowSpecWin11 first, followed by HighSpecWin11.
LowSpecWin11 took about a minute to restart Syncthing and then about another minute or two to show everything as Up to Date. HighSpecWin11 was about twice as fast. I did not use a stopwatch 
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HighSpecWin11’s database directories went from 449 MB to 746 MB. LowSpecWin11 went from 338 MB to 776 MB. The two Win11s have the same Folders being synced with shared ignore patterns and I installed Syncthing on them the same day, so I was mildly surprised that the sizes were so different. Unless some reason to do so develops, I won’t investigate.
No data loss, no errors, no sync conflicts, nothing weird or irritating to report.