I don't think it makes economic sense to Zero to offer a cheap upgrade path to the IPM motors even if the actual cost to do so is low. They are in for the money, not so much for the love of motorcycling is my feeling... That said, I think that they should be able to price it in a way that would allow those who really want to upgrade and keep their old bikes to do it in a cost-effective way vs. buying a new bike, but not cheap enough so that they would cannibalize new bike sales. Kind of find a happy medium where it will prompt some to upgrade and others to buy new.
For example. If I had a 2014 or 2015 SR and wanted an IPM version of it now, if I were to sell it, I figure I'd loose at least $5K if not more. Let's also say taking a $5K hit on resale seems too much to me - so I'd rather keep my old bike and not buy a new 2016 model. But if I could upgrade for $2K, maybe I'd do it. I'm sure Zero will make money on such IPM upgrades, where they won't make any money otherwise...
Of course, I might be terribly off in my logic...