Keeping its twentysomething fans on their toes, TeenNick is replacing two shows in its popular The ’90s Are All That programming block. Beginning Sept. 6, repeats of Hey Arnold! and Rocko’s Modern Life will run alongside All That and Kenan & Kel. Gone for now are Clarissa Explains It All and Doug. The late-night mix runs weeknights from midnight to 2 a.m. and, for the night owls, replays from 2 to 4 a.m. TeenNick chose the two animated series after polling viewers about their favorites via social-media sites. For the uncool over 30: Rocko is an Australian wallaby and Arnold is a fourth grader with a football-shaped head. See what you’ve been missing?