I run a few DA boxes on freebsd 7 but all these boxes were installed with 7 on the installation.
I expect the procedure is pretty much the same as going from 5.x to 6.x which I have done many times.
To minimise potential problems do the following.
Recompile all ports if you going to wipe old 6.x specific libs from the base system. Then wipe the libs after.
Install compat6x port.
Directadmin and its dependencies apache etc. all should work, ideally tho you should then chance the freebsd licence to 7.0 and force a DA upgrade, and recompile all the dependencies like apache, php, curl etc so they all linked to the 7.x libraries instead of older 6.x ones.
7.x is defenitly faster than 6.x and can handle heavier sites better, especially when you utilise the ULE scheduler on multi core systems. ULE will be default in FreeBSD 7.1.