Hello,

I realize you are concerned about the cost of ownership, and I also am not a huge fan of MS as well, but I don't think you should forget people cost at this point. If you were starting out from scratch I would say go open source all the way, but I assume (dangerous to do so) that you already have sys admins for the MS eco-system? You probably have a guy that is an Exchange admin? By replacing the legacy system, as opposed to upgrading it, you will also require extensive re-education of existing workers who maintain the current system to be able to maintain the new system that you propose.

What are your time-lines? How soon do you have to upgrade or replace? If you have the time, money, and people then I would say go for it. Just don't forget the retraining of the sys-admins and the users.

As you say, it is risky but doable. It depends on how much time you have until it needs to be finished.