Can I change a clip to proxy media if it's already in my project timeline?

I'm halfway done with a basketball highlight video, and I got a message saying to use proxy because "video frames were dropped during playback."

So I learned how to transcode clips to proxy, but I can only seem to be able to do that if the clips aren't in the project. That would mean I have to start all over again... Please tell me there is another way! Thanks