By structure I mean how do we price the trade-in value and how do we keep everyone (dealer , Bryston, customer) happy?
I guess that depends on how guilty you feel.
How many people either bought an SP2 or held off selling an SP1.7 at a good price because you told them it would be ugradeable? How much money did they lose because they believed you?
If you want to keep customers happy then I think the minimum you need to do is eliminate the financial risk of waiting for the final announcement. What that means is that you need to establish what a reasonable price is for a second-hand SP2 or SP1.7
now, and guarantee that it will
still be worth at least that much (as accepted by Bryston in part-exchange for an SP3)
after the SP3 comes out.
If you feel more guilty than that
then you should fix the trade-in value at what the second hand price would have been at the time when you first started telling people that the processors would be upgradeable.
I would also strongly urge you to consider dealing with the customers directly on the upgrade deal (in exactly the same way as you already allow SP1.7s to be sent directly to Canada for upgrade rather than requiring customers to go through their national distributor). For a British customer, for example, the opportunity to acquire an SP3 at US dollar prices would be very valuable; normally we pay double what the americans do. (Literally double!)
Btw, when it comes to possible upgrades, if there's no other option then I hope you'll consider a 2-box solution: a single new HDMI (or proprietary) digital connector on the SP1.7 or SP2 and an external box which has the job of separating HDMI audio and video, passing the audio to the processor, passing through the video to an HDMI output, and possibly also switching between multiple HDMI inputs. This would at least eliminate some of the concerns about a lack of space inside the chassis and on the backplane. (Obviously this is a last resort).