What if I backed up wallet.dat, after that create some new addresses on active copy of wallet.dat,
then run bitcoin-qt on old wallet.dat? Will I see my new addresses?
Maybe.
Your wallet.dat has 100 pregenerated addresses unless you changed that with keypool=X [1]. So if you backup and generated address number 101 you have a problem. If you had 99 or less addresses allready shown to you, you are fine.
[1] Note that you need to unlock your wallet for this to work.
I had no idea 100 addresses we're pre-generated. Thanks for the tip bro