Changing underwear is unnecessary, yes, but why would you get rid of the undershirt? Just wearing football pads without a shirt on isn't really the most comfortable experience (unless it's really really hot out).
Because after having a battle with a mountain of dudes, it's strange to see them all having the same shirt after stripping them of their gear. The same underwear on everyone for that matter. This has been stated alot in this topic so far.