My XJR's headlining had clearly been sagging for ages, as there was a big greasy mark above the driver's seat where his head had been rubbing against it. Loathe to spend £200 on a new board when the car had other needs, and not wanting to risk soiling or damaging cloth when fitting, I bought some grey perforated vinyl cloth and set about doing it single handed with the minimum hassle. With a bit of fiddling the old board left the car via the front passenger door, had to bend it a bit but surprisingly it didn't break. Then I scraped the foam crud off the board and used spray glue, leaving the edges to allow the board to bend on the way back in without creasing the material.
Worked quite well although the area around the light binnacle is tricky because of the curved surface of the sunvisor recess. But its a considerable improvement on before.
I also did it on my XJ40, the only difference being the screen side trims had to be removed and those horrible trim clips all broke of course. The plastics on the 40 are generally horrible and brittle, makes you appreciate how much better they are on the X300.
If you put plastic sheet down you could probably do the job in situ, but cleaning the old board would be fiddly and messy and I wouldn't fancy trying to spray glue upside down and trying to line up the glued material and getting it to stay up. It would be like trying to wallpaper a ceiling!