Building Expectations
One difficult aspect of overhauling an application is the loss of a preexisting mental model held by users. This is to say that users have developed an idea of where items are located and how they generally look and act. Often a major overhaul will cause these things to change a great deal.
Form the users perspective, one day they log out from an application they may love or hate and the next day the log in to a completely new application. Sure the data may all be the same and the end goal of the application will probably remain the same but for the user this is effectively a completely new application they must now learn.
There are several ways that an organization can bridge this gap and make the transition a little smoother. One way that I have recently seen and is quite simple, is just placing a notice on your screen announcing the upcoming changes. This should link some final comps of the upcoming look and feel. If you have a bit more time you can do what google did and offer the user both interfaces during a trial period. This method allows the user to become familar with the new interface while having the ability to fall back on the trusted version should the need arise. The danger with this is many users will never try the new version.
No matter what method you divise it is important to start building the expectation of change. This will give the users a chance to prepare. If done correctly it can also create a bit of buzz as users discuss what is to come.