if doc has advised better get it changed

Usually you don't require a change except for certain cases. So the most common reason a change in cast is required is if it isn't fitting well or its structurally compromised. Usually synthetic casts hold up well but if you've been called after 10 days it may be to take a look at the swelling. If any swelling which was present earlier has reduced the cast will have to be changed as the "hold" would have reduced.

If your doctor feels it needs to be changed then go for it. He knows best about physical condition of your arm.