Bootstrap itself is designed to be "themed" and changed in multiple ways, often with a very few changes in one file. You might want to explore that before you change out the entire framework. If you do go that route, you will have to change all of the classnames (and possibly HTML structures) at every level of your application's views. This is a non-trivial effort for an application of any size.
Or you could google for bootstrap themes (based on the version you use) and you can find some professional themes that change more than colours but you could keep your existing structure (or change it a little bit) to use them.