Changing a standard ceiling pendant for a new decorative fitting can be difficult depending on the type of new fitting and how well or poorly the original pendant is wired in the first place.
As a rule, there are normally three cables at a ceiling light fitting, that means when you remove the shroud from the rose there should be three red (or brown) conductors, three black (or blue) conductors and three bare cables (earth).
Of the three pairs, one will be the supply from the consumer unit (or the previous fitting in the circuit), another will be the supply out to the next light fitting on the circuit, and the third will be the switch wire. The important thing to remember is that the Neutral Conductor of the switch wire (coloured blue or brown) is actually a live conductor when the switch is on.
The switched live will be connected to the live of the pendant and should be sleeved to show it is live. To swap the fittings, first isolate the power. Next disconnect all wires to the pendant and remove it from the ceiling making sure that you have correctly identified the switched live. Join all the live conductors together with a suitable connector block and attach the new light fitting to the ceiling in accordance wit the manufacturers instructions, connect all the Neutral Conductors (there should be two) to the neutral on the fitting, switched live connects to live and finally connect all the earths together and join them to the appropriate connection on the new light and reassemble the cover.