How can I make thicker interior door slabs work as replacements for thinner slabs?
You can't (practically speaking) make your slabs thinner. It would be a monumental undertaking considering the nature of hollow-core doors and symmetrical mortising. You can either move the stops or get new slabs and start over. 

To move the stops, slice the paint with a utility knife, then work a steel putty knife behind the stop to loosen it. Gently pry the stop loose. Sand, fill, and paint the jambs. 

Determine whether the stops are salvageable with sanding and repainting. Reinstall using trim nails and filler, and apply a final coat of paint.