Always, mark and record things as you remove them - then if it is non-standard you can at least get back to operational.
In conjunction with the reference diagrams you will see there are live and neutral coming in, wires to and from the switch, and on a good day live and neutral going onwards to another fitting.
So, you will need to duplicate the wiring that is there, but if there is no room in the new fitting, then either a ceiling rose, joint box or one of these
(choc-box plus terminal strips ) will be needed in addition to the terminals on the new fitting.
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