Rainman76 more often than not a dealer is going to resell a used gun at what the wholesale is on a new one. With that in mind you are pretty much going to have to pony up and add some cash when trading a used gun for a new one. Depending on brand, condition and original cost there is a chance you can do an even swap but considering used Glocks for new Smiths I would say you're more than likely going to need some cash.
I have had trades before where a used Ruger revolver was an even swap for a used Smith revolver but it is certainly not common.
Good luck with your transition though. I'm carrying an M&P.40C for work and absolutely love it. The previous service pistol was a Glock 23 and that was purchased from the dept. when it was retired and sits in the safe for the most part. I occasionally pull it out to remind me how well the M&P shoots.
ENJOY!