You can now trade in your old Surface and you could get up to $700 toward the new Surface Pro 4. Surface Pro 3 devices have decent trade-in values. For example, you can trade-in 64GB model for $350.
Calculate the trade-in value of your Surface here.
Other conditions for exchange:
Valid 10/26/2015 until 11/7/2015. Available at Microsoft online store in the United States (including Puerto Rico) and Canada. Exclusions may apply. To receive full store credit towards the purchase of a new Surface Pro 4, customer must trade-in qualifying device subject to these terms. To be eligible for trade-in, product must power on battery must hold charge and not be required to be plugged in to operate, and be in fully functional, working condition without broken/missing components, cracked display/housing, liquid damage, modification(s) or have device warranty seal broken to be considered working. Cannot be password protected, and include original chargers/accessories. Any appraised value will be determined at trade-in and provided as a Microsoft online store redemption code and is valid online only. The online redemption code will be sent via email once CExchange receives and appraises your trade in.
Microsoft Surface Pro 4 went on sale earlier this week and you can order it from the below links.