It is better for the differential to change the oil once to many...I change the oil of my differential (and manual transmission) every 50.000km (30.000miles).
It is easy to do it yourself as long as you have a your car lifted up. There are 2 plugs in the differential. The upper plug is to check the oil level and fill the differential. The lower plug is to let of the oil.
with a minor 600+k