The DC website has had size large diapers expected to be in stock some time around Feb/March of 2015 posted for some time now. Are you talking about the new graphic design on the landing strip being available on mediums aslo? I know the larges will have the new design, not sure about the mediums, I'm guessing not till the current supply of mediums are gone and he orders more..?? I doubt he ordered a ton of new mediums when he still has a large quantity of first production mediums to sell (not a cheap investment). I could be wrong of course and the first generation of mediums will be clearanced priced to sell (like Xp's first gen of Absorbency plus).
To quote from the website:
Update: I have paid close attention to the public reception of this product, and I hear you guys loud and clear. In the future productions of this design, including the upcoming large, I will be replacing the anime element on the front banner with something more suitable to the theme.
To me it sounds like mediums will change in the future when he needs to restock his supply.