The biggest change will be that you might find it sometimes harder to get exotic hardware to work and that BSD tends to put stuff in different places. Most of your other unix knowledge (userland) will transfer just fine, the sysadmin stuff is a bit harder but learning another dialect of unix is along the lines of learning Spanish when you already know Italian.

FreeBSD always was targeted on running on servers. So if that's where you will run it then you should have minimal issues with drivers. If you plan to run it on desktop it might be more challenging.

Regarding sysadmin, I tend to disagree, I think it is the opposite, mainly due to great handbook they have (http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/).

As suggested reading "man hier" is a great way to understand the filesystem layout. I personally also really like clear separation between the system and applications. For example all ports/packages are installed in /usr/local including configuration files (in /usr/local/etc).

FreeBSD is just much more organized, Linux is more of a hodge-podge.

Read the hier man page of your chosen BSD, first. It is a great eye opener. From there OpenBSD and FreeBSD have some great documentation (not sure / no slight intended to NetBSD or Dragon Fly BSD as I have not used them much).