As long as the old equipment is still in good working order (not bent or anything), I see no reason why you could not continue to use your old WD set up.
If you knew the rating of the bars, that would help determine if you new trailer will need the higher rating or not. I did a very quick guess on the Equal-i-zer web site and punched in your numbers. They recommend the 10,000 pound bars for your trailer.
If your current bars are rated at 10,000, I see no reason to change them.
By the way.... the dealer will not "give" you in the deal a new weight distribution system. He may put it in the deal, but you'll be paying for it, either cash up front or in your loan. They don't give them away.
Good luck.