[jolmg]

If you want it as pain-free as possible, optimize for the transition once it's ready, and so avoid making improvements over the UI. Leave UI improvements until after people are using it.

I've been working on rebuilding a legacy system for the last "few" years, and now that it's mostly ready, we have a nightmare in figuring out how to transition the users as seamlessly as possible. They can do everything, simpler, but it's different, and will require training. Improvements would have been a lot better to do piecemeal after they've started using the new codebase. After discussing it a few months ago, we came to the agreement that we really should have done a port that resembled the original as closely as possible to ease the transition.

Too late to back out of it now, though.