We've taken over a legacy web application that has been running without anything changed for the past 4 years.
We are currently re-writing the application because the original source code has severe problems that make it easier to just "throw it out". The rewrite will be based on components from a very similar web property and thus:
The layout will be completely changed. Some deprecated features will be removed and all other existing functionality will re-implemented but may work slightly different that what's there right now. No major changes in workflow. The change will encapsulate some copy changes to reflect that we have taken over the property.
We're of two minds as to whether this transition is too major to do from one day to other with regards to user experience (browsing the website today and seeing what's been seen for the last 4 years and the next day seeing something completely different) and whether it's a good idea to "ease" users into it somehow.
Are there examples of such major successful migrations of large web properties? What can we learn towards this dilemma? Any other suggestions from experience?