Start writing new features with the new framework. Rewrite the most used parts of the old one, as much as you can. You will end up with a codebase with two fra

Start writing new features with the new framework. Rewrite the most used parts of the old one, as much as you can. You will end up with a codebase with two frameworks. It's not the end of the world. When we faced this issue, we did a complete rewrite. It actually reduced the amount of code and made everything nicer, legacy code is often full of hacks and undocumented nonsense.