I have worked on several projects(line of business web applications) in my career that involved a complete UI overhaul. Most of these projects were enterprise web applications that took a substantial amount of time to re-design. Some of them were done with a complete swap and some done in phases.
I was surprised by the reactions of the customers. If I had bet on the outcome I would have lost my pants. The wholesale swaps caught a ton of grief from the customer. The design really wasn't the issue, it was that it changed period. The phased changes went smooth as silk but looked strange for a while. A very undesirable UI for the interim.
Is it better to do complete re-designs in phases or a complete swap with the new design?
If dealing with a phased approach, is it worth the trouble to maintain both designs for a while, allowing the user to choose which want to use?
Even if possible, is it effective, or will the same problems surface when the old version is no longer available?