SAMBA 4 Forest Functional Level

Starting with Windows Server 2008, the goal is to allow raising the DFL/FFL without auto enabling any features. All of the features introduced in Windows 2008 based AD can be used by merely raising DFL. This means admins can raise DFL/FFL knowing they are only restricting the OS choice of newer DCs and preventing any downlevel DC been introduced.

So once you raise your DFL it wont auto enable anything. You can go ahead and start choosing when you want to use features like fine grained password policies and dfsr for sysvol replication. Other features such as restartable AD, RODC, AD Powershell etc are not dependent on DFL/FFL raises so you can use them already. See following links for details

2008 - http://technet.microsoft.com/en-us/library/cc753516(WS.10).aspx 2008 R2 - http://technet.microsoft.com/en-us/library/dd378796(WS.10).aspx

also see http://blogs.technet.com/askds as they have details on usage of these features.

I don't know where you're getting your information, but I admin a 2003 FL Forest and Domain, and joined a Server 2012 server to it a couple weeks ago without any schema update. I'm in the process of standing up another Server 2012 server onto it at this very moment, as well.

There's no schema update required to add a Server 2012 member server to a 2003 forest and/or domain. It's no different than joining a 2008 or 2008R2 server to a 2003 FL domain, in fact. Build server, join to domain, reboot and done.