The BD + DC version trades in for up to $7.60 at Amazon, if you already have it.
This release would be a 3-disc (3D BD / 2D BD / DVD + DC or 3D BD / 2D BD / DVD / online on DMR.com website for DC) or 4-disc (3D BD / 2D BD / DVD / DC) release, so there's nothing stopping you from picking it up now or later and selling/trading your existing version.
There may be a trade-up/upgrade coupon coming from Disney or a non-upgrade coupon as well, though we have some time before such a thing gets released. Amazon normally releases their coupon on these types of releases very early in the pre-order cycle, so it would be worth keeping an eye out for them. In the case of Tangled, the 3D combo pack had a $8 coupon on Amazon ($5 B&M coupon, so a better coupon deal) as well as a price match to the cheaper price at Target of $24.99, so it was a win-win.