You answered the question yourself really, you have a mixed enironment, which NOvell has no problem with. If you went to MS through and through you are opening a big, expensive can of worms, good luck as a sole netadmin for two locations!
Perhaps you can explain to them, if they had brains instead of wallets, they would be keeping the Novell GW system in place, upgrading to newer software versions and retaining the security and manageability of he system. Novell has no problem with a Windows server in the mix, not so the other way around though, why the change?
Should they choose to swap out to MS servers, they will need to make the hardware investment as it is not as resourceful as Netware.
I still don't see a valid reason for swapping the servers, explain that they will most likely need to hire another tech too as you cab't be in two places at once. Novell offers better management of both servers and updates, including the MS server with the accounting program.
What were/are they thinking?!?!? This is a law firm, not an ebusiness, perhaps you can explain the benfits of GW encryption and security, then show them how many Exchange targeted exploits there are.
Law firm email is not to be taken lightly, I have done my brother's fairly large and reputable law firm myself, this is no joke. Certainly not a place to start making backward changes in client email! They would even be beter off with a Mac Python email system in that case, and I wouldn't recommend that over GW.
Show them all the neato toys that come with GW 6.5 or something, that'll get em, secure collaboration online, one of the most popular features of my brothers firm!
This discussion will soon become a pissing contest and lose all focus but try and just read between the lines and take it for what it is.
This is like a bank deciding to stop using Brinks armoured car service because they heard that Terry's Tough Volkswagon's offer it too.