Communication is the key
When you start planning your migration, make sure to put your end users in the loop right away. Let them know you’re about to change their actual situation and try to get them to understand the reasons why you are doing it. Most of them won’t really understand or care for the reasons you are doing this, but in some cases you might get some unexpected feedback.
IE, once I had a group of 20 users working on a project using SharePoint for their documents. They were running at around 50 Gigs of content, nothing major here, but while talking with one of them about the migration, I realized they were referencing files to other files using internal hard coded hyperlinks.
Without that feedback, I would have never been able to address this situation in the planning phase and I would have ended up with unhappy users. I’m not saying you should tell every user everything you’re doing, but I’m suggesting you include a communication strategy in your project plan.