Removing a spotlight and adding a ceiling pendant - big job?

The scope of work would be the physical removal of the existing spotlight and disconnecting it from the wiring. That's fairly straight forward. The hole left behind would need to be patched in some way. That's also fairly straightforward. However, the wiring then needs to be re-routed and adapted to suit whatever light fitting you put in its place. This could be a simple job if it is close to the existing one and within the ceiling joists, when wires can be fished along using rods and a hook, or it could be difficult if the new light is the other side of one or more joists in the wiring route which block the route of the new wires. Carpets and floorboards may need to be lifted above the ceiling to get access to the wires, or bigger holes made in the ceiling to get to the wire to route them to the new position. If you have no DIY knowledge at all it is best left to professionals.