Like-for-like oil replacement is the smoothest boiler job in the trade: same fuel, usually the same venting path, same distribution. The day runs in a fixed order — assessment (heat-loss calc, chimney and tank inspection, near-boiler piping review), removal of the old block, near-boiler repiping to the new unit's spec, burner setup, then startup and combustion tuning with instruments. The tuning is the part that separates installs: a burner fired and tuned to the manufacturer's spec versus one just switched on.
Two things worth raising before the day: the tank — if it is 30-plus years old with unknown history, replacement alongside the boiler beats an emergency tank job later — and the chimney, which should be inspected before an atmospheric unit vents into it for another twenty years.