If you application’s front end runs on a commonly used UI framework and the framework is used properly it will be quite easy to align you application with Google Material Design principles. If not, converting your app to Material Design will take time, depending on the scale of you application.
Success Story: “In our office we had a relatively very stable web application which was developed for several years. However, the look and feel of the UI was too old, dull and had many other issues. It was a Java application and the front end was built using Bootstrap and jQuery. Initially, the request was to do minor twists to enhance the user experience and eventually we decided to align the application look and feel to Google Material Design. The feasibility study on the app realized that the frontend framework was used quiet well and the code was structured properly and the existence of Material Design plugins for bootstrap. Since this app was on production, we created a separate branch and applied the new UI changes. In less than 2 weeks, we were able to convert all the pages to the new look and feel and within a month, we were able to close defects caused by the new plugin. For an app which was developed for several years, making drastic UI changes within such short period of time with no critical issues was a great achievement by the team.” – MRS Team
If you have aligned your application with a proper frontend framework, it will be relatively easy to convert your app to Material Design format with minimal effort.