So, I'm probably way off, but maybe at least part will spark something with someone else.
I should begin by saying I am a word
Update thanks to suggestion from Reticality. I am a word, one word. Turncoat is one word.
In the beginning I followed established conventions
In the beginning a turncoat is a "good guy"
After that I decided that was not for me
Decided to change sides.
So I decided to oppose everything that came after me
Turning changes direction, possibly opposing?
In the end I decided not to oppose, but to protect
last part of turncoat: "Coat." Coats protect from the elements
covering, protecting and providing warmth
Coats provide warmth, covering, etc.