Here is the list of pre-orders this week with quite a few things that are "Available while in Demand". These are items that are not permanent products. See below. So without these, it appears you will have to order upgrade sprues later on. This would mean difference in cost as well.
via anonymous sources on Faeit 212
Do note all the items that are "Available while in demand". This means they are not permanent products, new to range means they are permanent.
So it seems GW wants people to order the upgrade sprue later on down the line instead of combo boxes. Noticed it's the same with the Blood Angels kits released last week.
