The Sefer HaIkkarim (Rishon) discusses this very question long ago, and he brings proof from Rabbi Hillel that one is not an apikorus if one erred honestly. Nebech an apikorus is not an apikorus.
Another thought that comes to mind is that on the Rambam in hilchos Tshuvah who writes that someone believing that Hashem is physical is a min, the Raavad comments that many of our greatest believed so in error, and they are not apikorsim. So the Raavad, at least, certainly holds that nebech an apikorus is not an apikorus.
Perhaps the Raavad does imply that the Rambam feels that nebech an apikorus is also an apikorus. Otherwise, what is his objection?