An original decorative “ceiling rose”…. Nice feature.
Likely the joint for the flex is above the ceiling and just drops through a hole. Possibly not in an enclosure though.
Is there easy access directly above? The floorboards will probably be cut to form an access hatch and screwed down.
Be prepared for a lot of dust.
Replacing the flex would mean trying to screw a modern fitting to that, which isn’t going to look great. It won’t sit flat for a start.
The screws would have to be long enough to go right up through this rose, through the ceiling which will be lath and plaster, and hopefully into solid wood.
what sort of fitting would you like to replace it with? Just a flex with a shade, or something grander, bigger and very much heavier?