Phasing out BBCode and going for all-HTML down the road is a good thing, it just needs to work correctly.
People get all sorts of annoyed when they go to edit a post and some new behind the scenes change which they don't care about totally messes it up and causes a load of work for them. Also in general, most users hate changes and it discourages them from posting because they don't want to bother getting to grips with something new.
The most important thing though, and it's been said by everyone here, is it needs to have total backwards compatibility. Parsing some BBCode into HTML but they might not turn out exactly how the BBCode displayed things? Then don't parse them and save it that way, you need to save the original so that the content as it was intended to be displayed isn't lost because of some screw up made in conversion.
I always love to upgrade our forums and get the latest features, but the biggest worry for me is always that some change that has been made (just like this) might cause our users to dislike the forum and thus posting will go downhill. It's also not IPS that takes the blame when this happens, our users hold us responsible and we suffer for it as well.