Controversy brings traffic and with traffic comes revenue. You might not want to delete any hubs because you might have a url that is popular in searches and you wouldn't want to give up that url to someone else. Usually if a hub is getting good hits than they are working for the hubber. Even if you are going to switch over ot only two main subjects keeping your old hubs might be a good idea just to keep the hits coming in on them. Perhaps you could link your new hubs to them to draw traffic around you work. I was going to delete hubs that had nothing to do with what I normally write with at one time, but someone also told me that the url if it is bringing in traffic might be a good url that is popular and since you have it drawing the traffic in you can still get hits on those.
You also have to take into consideration that your current hubs are cached and paged on google which is giving you (and your name) apperance around the web.
Though it is a personal option on what you wish to do, you have to think if you only want those specific key subjects only and not your old hubs floating around.