Oil change Corsa gearbox?

Hi, I would like to change the gearbox oil on my "M" reg Corsa automatic. I got under the car yesterday and saw the drain plug and what looks like a filter that looks like is secured by a circlip.Is it an easy job and can I do it myself? It looks easy enough.Are all filters available from Vauxhall and is there a diagram of the gearbox anywhere on the web?
Cheers
Eric