The Harry Potter Plagerism Issue

As I'm a full grown man, I have little knowledge of Harry Potter other than catching parts of the movies on cable or on flights. But the other day, I saw something worth noting about the books/characters.
I came across the cast list for the original Battlestar Gallactica series. I saw there was a kid in the series and wanted to see what became of the actor (since other than Jane Seymour, the only actor I recalled was the guy who played Face on the A-Team). So I clicked on the link of the child actor then, Noah Hathaway, and saw that he was in a movie I remeber as a lil' kid, Troll. What was odd was seeing that the character he played was Harry Potter...and the movie was from 1986.

Huh?

Yeah, Harry Potter was around in 1986. He was a kid who learned about his hidden gift for magic. So I just assumed that the writer of the Harry Potter novels wrote the Troll screenplay. Nope. She just lifted the character form Troll.

Some minor research into other plagerism charges show JK Rowling based the entire series of novels on something that already existed, The Worst Witch, again from 1986.

Well, I gave this person too much credit for building an empire out of an orignal idea. But perhaps if I had more background info on Harry Potter, I wouldn't be learning this now. I'm sure this is common knowledge the fans.