So, for example, try running something non mission critical such as the Alumni database, or a webcasting server or a software mirror service on a FOSS box. Then people start to say how fast it is/doesn't break down. Then try something with some bite in it - maybe a read-only Samba server. Then maybe a read/write Samba server for a small group, which is linked to AD. Make the move in small steps, a bit like the Embrace & Extend strategy.