After the vision has been set, you need to plan for your migration.  Some of the key decisions / steps are highlighted in this checklist.   You should make sure to complete all the items of each step before proceeding to the next, even if the decision was to simplify, effectively putting your web site on a diet.  Also, please check out Five Suggestions for a Successful CMS Migration.  

Note that it's a bit tough to isolate articles that are specifically about planning, since ideally you're keeping in mind what is coming in the pilot, implementation, and training when your are planning for your migration.  That said, the articles at the right should be particularly relevant to your planning.