I've never even heard of Joanne Kelly. And to be honest when I get attached to a character in a show, then I get attached to the actor playing the character. I wouldn't have been able to continue watching the show if someone else took Shannen's place. It's ashame that they killed of Prue but I would have rather have had that than someone else play her. No one could play the role of Prue like Shannen did. But that's my opinion.

It'd be weird seeing another actress portraying the same character. That only works in very limited circumstances. Although that's what they've effectively done in the comics, I think it'd have been too soon to do it in the TV series, particularly immediately after Prue's death. And from how I understand it, Shannen's departure had been so bitter that they couldn't really use the character in any way.

Come on! Nobody watches Warehouse 13?! Joanne Kelly plays Myka the nerdy warehouse agent. Watch some episodes you crazy f#chers!

^I have watched Warehouse 13 (though I didn't make the connection with the actress' name). It doesn't change my opinion about changing actresses.

Most of the time, changing actors only works for minor characters where the majority of viewers might not even notice that it's a different actor. There are a few famous exceptions - Dr. Who & James Bond - where changing actors has become pretty much a tradition, and in Dr. Who they've written an explanation for the change into the show's mythology. But since Charmed does include magic, they could have possibly explained a change in her appearance as due to the effects of a spell - they did something similar to cover Paige changing her hair colour in between seasons.