It sounds to good to be true. It will be one hell of a big job.
Also I think when the rework is being done a feature / release freeze needs to be placed.
So you don't have to rework your rework with the updates done.
When there just a few people doing the rework I think that the freeze will a be for a few months. Maybe shorter when somebody works his *** off (which would be greatly appreciated). But the main programmer is always Guino. I guess his opinion will be decisive.
Apps always tend to get slow when al kinds of extra features are build in ad-hoc. You will get a eight headed dragon with a slower reaction time.