I agree with @Metalmuehle and @Kae, just offer one additional product (e.g. 1000kcal) in one flavour (e.g. vanilla or perhaps the neutal flavour people have been asking for) and let people mix it however they want with the other PlennyShakes, or blended with other foods. It should still meet the same RDA as one packet of PlennyShake, but with less Calories.
You could even hold a naming contest in line with Plenny and Twenny. BlennyShake* for blending? SkennyShake* on the skim side? Or just HalfplennyShake, since a halfpenny is a thing.
Quite a few of us I’m sure are taking less PlennyShake than we should because we don’t burn 2100 kcal a day.
*If the ‘enny’ part isn’t crucial then BlendyShake and SkinnyShake may sound nicer