The humor of South Park is what makes the show great and has kept it popular for many years. The drawings do not really detract or add to the show that much. The story actually plays out in the viewers mind. The characters are recognizable, but come to life in the viewers mind.
Yes, the crude drawings are a big part of South Park, but the highlight of the show is the dialogue, and watching in high definition isn't going to change that. In some ways the drawings may even be better, as the line will be clearer and the color will be better.
No, watching a show that has been re-rendered will not ruin the experience. It will only provide a new experience that for some fans may even be better. Even if not everyone will like the new format, I'm certain there will be many fans who will appreciate re-watching South Park in a new way.
South Park is a cultural success because of the commentary of the show, not because of the crude drawings. It's obvious from the first time you watch an episode that South Park isn't very well animated by historical standards. As long as the jokes are the same and the characters are the same, viewing the show in HD shouldn't make it lose any of its luster.