If I understand your post right, the brick is already painted since you're asking how to prep the 'painted' surface. Bummer. (I happen to really, really like the look of brick.)
If you don't get any help here, stop by a local paint retailer like Sherwinn Williams or Finneran & Haley or the like. You may not get too much help here because most previous posters would answer a post like yours by saying don't do it (assuming the brick wasn't painted yet). Actually, I don't ever recall anyone requesting help for painting a previously painted brick home. Well, my best to ya and hope this helps.