It went like a charm. License upgraded on the first try. No problems whatsoever. Judging by my experience, it may be safe to go ahead and upgrade now.
One thing, though. I couldn’t get the Steinberg Download Assistant installer to download. I was using Chrome under Windows 10. Then it occurred to me to use Edge instead, and it downloaded fine the very first time. So it might be helpful to keep that in mind.
Cubase 11 looks good so far. Opened my 10.5 project with no problems at all.