I suppose this won't be known as Eeyore's Syndrome. Never mind, it doesn't matter anyway. Nobody cares.
This isn't a simple question.
It depends on the purpose for the redesign. The engineers probably have the best insight into correcting code & data inefficiencies.
Many people should probably have input on usability & functionality redesign. The UI and engineering people need a deep understanding of both how the app is currently used and what the goals of the app are in order to make the flow efficient. Most people resist change that will add efficiency -- accelerators in GUIs, changing the keyboard layout to something that allows faster typing, etc. Therefore asking the users won't necessarily yield answers that really improve the app, although the users are going to find at least the most glaring process issues, and sometimes a user will have a real insight. It typically takes asking the right questions after gaining a deep understanding the purpose of the app to sort out what are legitimate improvements as opposed to personal preferences (although accommodating those can improve efficiency too). Then the testing process can reveal whether something intended to improve the process actually does so.
A business application is a lot like a shovel. It's a tool to speed up a job. Yet as simple as the shovel is there are multiple interfaces (shovel shapes). This is because some of them are more efficient for some tasks. Even after all this time we see new iterations, such as the snow shovel with the funky handle that claims to relieve back stress. So make sure that you have someone with the skill of asking the right questions about the redesign, regardless of what group that person is currently assigned to.