Matthew has written a very comprehensive article about all the things that you need to think about and all of the questions that you need to ask before embarking on your messaging upgrade or migration.  He covers all the different parts of the planning phase, and the sort of questions that would likely be asked of different parts of the business prior to the start of the project. 

The article can be found here, and it's certainly worth referring to if you have a similar type of project about to happen on your organisation, and creating a checklist of things to consider.

By the way, if you want to submit an article for the Insiders here's how you can contribute.