Manual upgrade using base.txz

I'm sure a lot of people have FreeBSD machines running on older 
versions. In recent years, TPTB have removed all but the newest versions 
of anything from repos, which puts the kibosh on using freebsd-upgrade 
to to bring them forward in stages to a current version. After a year or 
so you're stuck with an old release. And I'm sure we've all had 
freebsd-grade go south anyway.

Assuming you know what you're doing with configuration files /etc/, what 
would happen if you booted from a CD and simply unpacked base.txz over 
the current configuration? Let's assume it's on ZFS or UFS2.

I've always had the idea that this installed the complete base system 
"factory reset" style. But I've never actually tried it.

For many years I've run everything in one or more jails, and IME, a 
jailed environment from an earlier release tend to run just fine on a 
new kernel, so I'm hoping the solution is as simple as I've always 
thought. But if it was that simple, why isn't everyone doing it?

Thoughts?

Thanks, Frank.