Andrew said
Just found this article, and apparently the old
displays were replaced
with the new ones shortly after the grounding in 2000/2001. This would
mean they are probably LCDs.
http://www.concordesst.com/returntoflight/mods.html
Also, here's a photo that very clearly shows the refresh:
https://www.jetphotos.com/photo/7009833
Interesting. Searching for 'Marrilite display' (I wonder what that is) turned up
this page, which mentions
"There was a plasma display at the front of the cabin showing the altitude, the air
temperature and the current speed in both miles per hour and Mach number."
https://www.heritageconcorde.com/concorde-cabin--passenger-experience