It also depends on how deep do you want to go with language features.

Let's say you have existing ES2015 source code. You could start experimenting on a local machine and use a script to rename js files to ts. Run the TypeScript compiler and annotate some of the most used functions.

You can then check what is the experience of using those functions that have type annotations, what is the speed of the development, etc.

Sit down with colleagues, show what you have done and ask what do they think. You can then decide to proceed and stick with type annotations until you're sure. Use boy scouts rule: "Always leave the campground cleaner than you found it." aka add typings to the function declaration.