I have several codes. When I have more than one code for a movie, I redeem one for myself. This one redeemed in HDX. But i don't want to guarantee that the rest will be the same. Especially when they all say "Offer may or may not include high definition versions." It's only when you redeem it will you know for sure if its HD or not.
In general, a code that comes in a bd or combo pack is "suppose" to be HD. But the studios also understand their might be mess up that's why I've never seen a voucher that said "This IS HD" they either don't say anything or say that it might or might not be included. Some ultraviolet sites will let you try to prove it came in the bd or combo pack so you can upgrade it.