It is possible to replace them with the intake in place, but you must have the proper (1/4-inch drive & deep socket) tools, and it's NOT easy. I recommend pulling the intake off, changing the plugs, cleaning the intake, replacing the intake gaskets (which ALWAYS leak around the #2 runner), and also replacing the IP vacuum shutoff if it is original (this job takes 5 minutes with the intake off and almost 2 hours otherwise.) Photos of the job on my sister's car are here:
http://www.meimann.com/images/mercedes/OM603_intake/
(I also tore off the ALDA tamper seal and turned it 1 turn CCW to improve off-idle performance.)
Regards,