Starting TDD halfway a python project, good practice?

I am halfway in the development of a large Flask web application.

Back in the beginning I was not willing to use TDD, for learning curve vs project deadline reasons.

Thus these last weeks I had the opportunity to learn more about it, and I am quite exited about starting to use it.

However, regarding I didn't wrap my project with TDD from the start, is there any cons of starting to use it now ?

I would not refactor my entire app, only the new features I am going to design in the near future.