When possible, it is better to remove the buffers by pushing them from the back. Caution: some spring buffer plates tend to escape. Remove the bogies (just pull).
To separate the chassis from the body, do not start by separating the side walls as indicated in the Vitrains instructions: this is the best way to break the small lugs that hold the body on the chassis and prevent it from splaying. It is preferable to first lift one end of the chassis by levering between it and the body with a small screwdriver and then to move towards the other end, moving away as little as possible the body sides.
Do the same to remove the interior fittings, whose side clips are small and fragile. The upper stage fitting is snapped into the panes, so just spread the sides of the body in order to disengage it.
Check the gluing of the windows (sometimes too much, sometimes too little glued), especially the small end windows. Personally, I re-glued the latter with liquid model glue that penetrates very quickly (beware of overflows!), while holding them well pressed with small spring clamps.
Note: in reality, the clamp is placed on the window, with a waxed paper between the clamp and the body to avoid their gluing in case of overflow. But that would not prevent the degradation of the paint! The glue is introduced with a brush at the top of the pane because it is missing there at the origin.
Break and/or trim the interior reliefs of the roof, in the axis of the car, which would hinder the installation of a light strip.