I agree with the above advice and would only add that even if transferring everything over including fish, you will still have a mini-cycle occur as it all gets re-established. It could be 3 days or a week, but you should still test the water to be sure all is ok.
also be certain the water matches the old tank water in terms of temp and ph otherwise you run the risk of harming the BB colony.
IMO you could use about 30% of the old water to get everything set-up and decorated, then top off with treated water. That way it is less risk of shock to the bacteria colony.
I also advise to stay away from bottled bacteria, it's a 50/50/50 proposition as to whether it will help, hurt, or do nothing. If it is past it's very short shelf life, it could foul the tank.
gotta remember those little buggers are the most important livestock to have in the tank.
edit: if the current tank water tests high for nitrate, >20ppm, then I would forgo using it and just be sure to treat and age the new water.