Your product is long in the tooth and you have no idea what your customer wants. So it always seems like a good idea to sprinkle a little UX dust on the product and wallah! Wow!
It is true that ensuring that the visuals of the product are current and modern will remove a barrier in the sales process. It is also true that a clean modern looking UI will go a long way for hiding basic usability issues. People will believe that the modern UI is better than others that look less modern.
That said, wow that is pretty, is not the sensation your users want to have configuring all the pages in the application. Nor are they looking for pretty while maintaining and troubleshooting their application.
Think about it. If you are the admin for an enterprise application is pretty the wow you are looking for? Or does this person want an application that makes his job easy and makes him look good.
Observe your users. Notice all the people and things they have to interact with. Make that simple and convenient.
What kind of time pressure are they under and how do their tools help or hinder them? What happens when they are late? What can you do to alleviate these pressures?
Observing your users will tell you more about their needs then any Wow facelift can ever solve.