[bzbarsky]

I'm not sure what sort of distinction you are trying to make here. Again, the two options are:

1) One big switch. Can probably be done faster, but has more risk that big problems won't be discovered until late in the game and has user-visible benefits until the switch happens.

2) Incremental changes. Take longer to complete, but allow for better mid-course correction and can show user-visible benefits much earlier in the process.

Which of those will lose more customers? It really depends on the market reality and at how successfully the incremental changes can be made.