I attempted to queue for level 10-19 WSG today and I leveled up to 20 while waiting in the queue. After a while the queue popped (it was the 10-19 bracket since my friend that was level 19, got the same one) but when I clicked to enter it just removed me from the queue without putting me into a BG.
My suggestion is that when someone levels up during the queue he should still go into the battleground for the previous bracket. I am not 100% if this was the case back in vanilla retail, but I can clearly remember it throughout TBC and WotLK. If this was not part of the game back in vanilla then please ignore my suggestion.
Thanks for your time.
-Tsoop