no offense 'tol but is this a 'real' DH bike? if it was it'll be an effin b!tch to climb with, i'm guessing the reasoning behind the 2x9? proper DH have less gears 1x7 and does not have a low gear like most MTB's. but if you still want to put a 2x9 in there, make sure: - it has a mount for the FD - the biggest/smallest chainring you can put in
and NO, it's not an upgrade nor a downgrade. it's just a — change.