source upgrade FBSD10 -> 11 (then 12)

[Lee Damon]

I have three old FBSD 10 boxes that I need to upgrade. Ordinarily I do 
this by building a new box with the latest OS then migrating services 
and data. Unfortunately I don't have that option this time, the upgrade 
has to happen in-place. My plan is to go from 10 to 11 then from 11 to 12.

I was looking at the "Upgrading FreeBSD" part of 
https://www.freebsd.org/releases/11.1R/installation.html#upgrade but 
unfortunately it seems to be missing a critical URL or two:

"The procedure for doing a source code based update is described in and ."

I tracked down where I think it points and the instructions appear to be 
just for same-version updates. Can I safely move /usr/src out of the way 
then check out the stable/11 and compile/install it or are there other 
things I need to do?

thanks,
nomad