I have done this, the dealer will typically give you trade in value what you paid for the machine new in a specific time frame. It is a good way to upgrade to the machine you need.
(But first I would try to deal for the new machine without trade-in to see what his bottom line is. They typically take the trade in value off the full retail price, which is more than they would take for the machine without trade-in.)
Good luck! I love my Pfaff for piecing because of the built-in "walking foot"!!!