Designing new interface for a legacy web application

Current situation:

We have a legacy web application. It is been in development for 5 years and already has many customers. It's a very critical and also expensive product and its customers are big organizations, including the government.

At the start of development, the team didn't have any UI/UX designer and each developer did all sorts of work, including back-end and front-end.

Problem:

Customers are not satisfied with the current UI/UX of the system.

Questions:

I'm supposed to address this problem. Now I have many dilemmas and decisions to make.

  1. Is it better to make changes on the current UI and improve it, or to design a new interface from scratch? (the current design is like '90s designs and making big changes is almost impossible, especially when changing the UX)
  2. If I'm better at designing a new interface, should I use pure CSS or frameworks like Bootstrap/semantic UI or use an admin template from ThemeForest?
  3. What's the roadmap for this change? Like, understanding the current system, sketching new designs, implementing a new theme, applying it to the system... Should I apply it to the system or should the developers? The current system used the classic approach for its implementation, I mean it's not SOA and it's not easy to change the theme. Plus, the front-end code is mixed with the back-end. (Django template builder)
  4. How to deal with customers? Bringing such a major change is kind of challenging and may have a bad effect on customers.
  5. Any other tips that I should know about?

Thanks for your time.

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