Not quite - the Didcot stopper basically becomes semi-fast east of Reading and the Readings stop more.
Currently Readings stop Ealing Bdwy, Southall (down only), Hayes & H, W Drayton, Slough, Burnham, Taplow, Maidenhead and Twyford; and Didcots stop Ealing Bdwy, Southall (up only), Hayes & H, W Drayton, Iver, Langley, Slough, Maidenhead, Twyford, Reading, Tilehurst, Pangbourne, Goring and Cholsey
In December the Readings pick up the Southall (up), Iver and Langley stops to become all-stops west of Southall (skipping 3 stops east of there). The Didcots lose them, only stopping at major relief line stops east of Maidenhead (Ealing, Hayes, W Drayton* and Slough only - skipping 8 stops) and then stopping all stops
*which will be lost when through-services begin on the Elizabeth line, with Maidenhead services meaning 4tph off-peak and so no need for Didcot services to stop there.