Was any operating system actually able to go from single-user to multi-user so easily? Windows NT and OS X were totally rewritten from the ground up relative to their single-user predecessors Windows 9x and classic Mac OS.

NT is closer to a mainframe OS than 9x and it came out in the win3 era

Yeah, I actually used NT 3.51 for some time. However it was the NT line that replaced the 9x line, at least from the consumer perspective.

NT had no predecessor (at least from Microsoft; architecturally it's predecessor would be VMS). It was ground-up multi-user. Mac OS [Classic] was not OS X's predecessor, either -- it was NeXTStep.

You are right on the technical lineage, but I was referring to how these products were (ultimately) presented commercially. The facts that they are both older than one might think and they were developed with specific goals from the start I think more clearly illustrate that you can't just "bolt on" such fundamental differences.

yea, a big one. Once there was an intentionally multi-user-from-ground-up OS called "Multics". Dude named ken thompson or dennis ritchie or something worked on it but was like "bro, so bloated". So dude writes a DISK BENCHMARKING system, single user, called it "Unix" as if to castrate multics. The original unix was disk benchmarking, barebones. So we know how that moved into multi-user

source: kernighans readingbook, "unix a memoir" or something like that, great read

I'm not sure how much that really counts. According to [1], the time that Unix spent as a single-user operating system was intentionally very short-lived. Even the earliest version of the operating system that can be reconstructed today had multiple logins and processes, though only one could be active at a time. It seems that, by the time Unix was written in C, ported to multiple architectures, and spreading outside of Bell Labs, it was already multi-user.

[1] https://retrocomputing.stackexchange.com/a/10908