I think it's important to separate feature improvements from a technical rewrite, ideally in the rewrite you mostly just make things work the way they did, sometimes you might fold a feature improvement into it but if you come out of the rewrite with a more stable product that has about the same usage stories you should consider it a success.
Sometimes you will want to fold features into a rewrite (remove prompting the user to confirm X twice) sometimes this will ease development and be worth it but other times it'll pay off to just retain the old functionality but add it to a list to be user tested later.
Once the tech is solidly over then take a swing at updating the poor UI, do it agiley so you can back out of changes that the user base rejects since (at least within my more modest usage studies) not everything people depend on comes up or gets reported. I'd much rather rollback a design feature branch then have users get change fatigue when you're forced to rollback your new shiny rebuild and the whole project ends up being shelved.
Sometimes you will want to fold features into a rewrite (remove prompting the user to confirm X twice) sometimes this will ease development and be worth it but other times it'll pay off to just retain the old functionality but add it to a list to be user tested later.
Once the tech is solidly over then take a swing at updating the poor UI, do it agiley so you can back out of changes that the user base rejects since (at least within my more modest usage studies) not everything people depend on comes up or gets reported. I'd much rather rollback a design feature branch then have users get change fatigue when you're forced to rollback your new shiny rebuild and the whole project ends up being shelved.