☆.A.
Well, I guess I always default to the "earlier the better" and "honesty is the best policy" things.
I would schedule time and sit with my supervisor and just honestly explain the big picture, including that you value your job and would like to keep it through the internship, if possible.
I might wait until May/beginning of June, especially of you feel the PT idea is not going to go favorably. But not much later. You don't want to "stick" them with no O..
Kind of hard to know what to suggest not knowing what kind of work/career you have right now, but I think scheduling a face to face....even beginning with telling your supervisor that this MAY be happening in the fall, if they can place you in an internship.