I've gone through this at a few places now. Besides resistance from the users ("we only know how to use Outlook!"), is migrating from Outlook to another solution ranges somewhere between unlikely to impossible. For someone like me, I only have 3 or 4 appointments scheduled, and the other few hundred are meetings I was invited to. :)
You can have the best plan, with the best business reasons, but when a senior executive tells the CEO that he can't switch, you'll frequently find that it will veto the migration.
Here's a real-world example. I was Director of IT for the company. The CEO told me specifically to get rid of Exchange, because the upgrade costs were too high. We were literally a couple weeks from switching. The Director of Sales went to the CEO and demanded that we keep Exchange, or he would walk.
Funny thing about the sales department. He didn't manage to sell anything, and he couldn't retain the customers. The accounting staff ended up doing all the customer retention. That guy cost us more money than he made. IT, on the other hand, brought costs down, and improved the customer experience.
The only thing that sales brought to us were headaches, and very pretty forward looking reports, that pretty much consisted of a graph showing our sales history, and a line going up at a 45 degree angle showing our future revenue. Every few months, he had to update the graph, so it showed our revenue losses, and had a new starting point for his upward line. I don't think he had a grasp of the concept of forecasting.