As I endeavor to go through my old stories, I'm finding quite a few things which do not mesh with my current books. I read whatever it is in my old books and go, "oops, can't fix that now". So here are my ideas on what should happen.
1) If you are doing prequels after your main story arc, go with whatever the change is in the published books. This can take the form of just switching to your new idea and having to work it in to the older stories. There were a few things with Amber and her having Richard as a Charge that do not happen in my published works with Miyaca and Shadizar.
2) Which this leads into making a reason that it's different in the prequels. In my case, my book timelines are thousands of years between stories, so there is definitely time for change. In the case of Amber/Richard she isn't supposed to let Richard know she is his protector on pain of death, (which, of course, does not happen) I just make it that after Amber's ordeal, the Clan Leader(s) decide it's easier if the Charge knows who their protector is. Thus, this explains why the difference between Amber/Richard and Miyaca/Shadizar situations.
3) The third item might be you need to get rid of the idea all together. I have a few, and it will definitely take some reworking to make it mesh with the published books. Their plot points so I don't want to give anything away...
Don't give up, just think of it as a writing exercise!
Happy Writing!