Google downlighter blanking plate, above the existing light should be a junction box for the cable, so that bit should be easy, the main problem is going to be supporting the new pendants from the blanking plate sufficiently to prevent them dropping onto the island, without a noggin in the ceiling spanning the joists to screw the blanking plate into you will be relying on the strength of the plasterboard to support the pendant, easy enough to angle a piece of wood into the hole to span the gap and spread the load a bit, but again it will be relying on the strength of the plasterboard to take the weight of the pendant.
When you pull the existing downlight from the ceiling take a picture of the wiring configuration for reference when you wire up the new pendant.