Converting 50fps footage to 25fps- best way?

I'm cutting together a sequence that I intend to be 25fps, but most of this footage has been shot in 50fps.

What is the best way to convert it all to 25fps? I am afraid that the footage will go choppy or look "off."

Is it better to leave it as is, and convert it later/ export it as 25fps when I have a finished product?