does changing one ceiling light effect the rest?

Yes, Tammey this is exactly what you have done - the loop in system goes from one light fitting to the next so the wiring that you see is in fact live even if the light is switched off. The wiring is like this...live and neutral come to the loop in rose on the ceiling and continue to the next one - so thats 4 in total. One of the negatives is tapped off directly to the bulb, but one live is tapped off to the wall switch and then returns to the bulb - only being live when the switch is on - so now thats 6 wires, not including the earths! Please take care here, and call a sparky in if you're not sure.

John
April 2009