How do I change an established 55 gallon from pea gravel to planted? Do I have to start all over, empty and put in substrate?

No need to start over at all! The easiest way would be to add root tab fertilizers directly into the pea gravel near the base of the live plants after you've put them into the gravel. Additionally the type of light you have might need to change. The easiest way to learn more on this topic would be to watch the Aquarium Co-op YouTube videos on the subject. They are incredibly helpful and easy to digest the info. Couldn't recommend them enough.

Depends on the types of plants. I've got a variety and just do root tabs/liquid aquatic plant fertilizer.

Thanks, I'm going to try it. I have a Fluval plant light.