Georgiw...
I have not admin'd a Mitel system before, I've always done Avaya and Cisco. But you're question is interesting to me: I'm in the opposite position: We are considering switching from an Avaya to a Mitel system, mostly because of Mitel's ability to virtualize.
Why are you migrating away from Mitel? What was it about Mitel that made you consider Avaya? Is your Mitel an IP-based system, or an older TDM-based system.