only replace what broke (if at all) or what is an upgrade. Going from 9 speed to 9 speed is not an upgrade. Go to 1x11 if you want, since that is the only sensible actual improvement. but evaluate first everythign else ont eh bike what needs replacement and make a bill of material what you will have to pay.
a 26" MTB is worth nothing, and adding $300 in upgrades still makes it 26". sorry, but that is the market. It may be worth more to you, but not if you want to sell it.