Thought I'd post this since the guides don't mention direct upgrades from Mojave to Big Sur (unless I missed it).
Having successfully updated from CaseySJ's OC 0.6.6 to 0.6.8 a couple of days ago, today I successfully did an in-place upgrade from Mojave 10.14.6 to Big Sur 11.2.3.
It took a total of 30 minutes and 4 restarts (the first one prompted, the rest automatic).
After two hours of testing apps, so far everything seems to be fine, even two music apps which aren't yet supported in Big Sur: Digital Performer 10.13 (though I only tested playback, not recording) and UVI Workstation 3.0.17 (which I tested only in standalone mode).
No problems so far with any other of my music apps, either: Pro Tools, Dorico 3.5, Sibelius, Metric Halo MHLink (Ethernet audio), Keyscape.
Of course, I made a bootable backup beforehand (I use Chronosync), so there was little to lose except some time.
Delighted at how painless this was! Thanks, CaseySJ and the OC team!