How to fix door side panel to existing frame

[thedoomed]

I'm looking to replace an internal door and side panel (pictured). The door itself is easy to replace and is a standard size (838mm) but the side panel will be a challenge.

The frame looks like it was supplied as a single unit including the side panel. There are no strips around it i can remove to then remove the panel. I plan to keep the frame - but may replace the central part of it.

Assuming I can remove the panel okay, how can i securely fit a new panel back in place? I can put dowels on one side/end, but what about the other two sides (other than no more nails...)?

 

[Notch7]

The side light is a fixed door, set in a frame.

My guess is that the frame was fitted first, then the sidelight was screwed in place through the glazing rebate, then it was glazed.

If you replace the central mullion you will find it is mortice and tenoned in place. The top tenon will be an offset shoulder to fill the rebated head.

Given that its an internal door now, Id be tempted to rip out the frame and replace with a liner. You could then have a 3 sided liner, so no threshold on floor to step over. Bear in mind, if it used to be an external door, then it should be replaced with an external door.........