This is the part of a rewrite that scares the crap out of me. When you have thousands of users across hundreds of big clients, with millions of existing records and a decade of history, making a big switch in one go seems impossible.

Since this was a rewrite of a system that just consumed the output of the CMS, the migration was not awful. One day, we just started exporting the new data format and using the new system to create apps. Users continued to use the same CMS to build their applications -- the CMS was the target of many improvements, but didn't get replaced wholesale. Thus the rollout of the new system wasn't really impacted by the number of users of the CMS. Though the Dojo/PhoneGap system in reality exists pretty independently from the CMS, you might think of it as the presentation layer of the CMS, so this rewrite really targeted a (significant) piece of a larger system.