My son is 8.5 months old and does the same thing, only it is not limited to diaper changes. It is also when he is being put into his carset, highchair, etc. He doesn't quite crawl yet, but can army crawl around really well and prefers to pull him self up to stand at tables, toys, etc. I think it is a stage as I seem to recall my 4.5 year old doing it too and is related to the new ability to move around independently.
Like the woman down below, as soon as he could stand on his own, I started changing my son while he was standing. It just was easier than fighting with him to lay down. I know my little one has a ways to go, so for now i just try to distract him by tickling him and singing to him and telling him that I am just changing his diaper and he can get up in a minute.