Hi Raffi. This is very specific question regarding commercial formulas. I am sure you know that. Anyway, I have some ideas referring to my experiences in managing a factory for a few months. 

I doubt that your assumption about 10% sugar is true. Currently, I think the included sugar has been reduced up to 6% in some brands (seen myself). Therefore, let`s talk briefly about the basics.  In yogurt, generally, there are sugars from fruit (fruit sugar), lactose and added sugar (sucrose).  It all depends on the producer and respecting brand to choose the amount of each. However, one should consider that reducing sugar must be done slightly because customers will notice it very quickly (self-experience).

Final Answer: Berries are the best source of fruit sugar for yogurt which also keep the healthy level very high.

Many yogurt brands have artificial sweeteners added such as aspartame and acesulfame as sugar alternatives. However, if you have concern over the use of artificial sweeteners then you can try natural plain yogurt without any added sugar. Many brands are available in market with little or no added sugar. However, lactose is always present as natural sugar. In Greek style strained yogurts some of the lactose is drained with whey. As far I have searched many brands contains 6 g sugar/6 oz serving size which is around 170 grams. Therefore, you can try any of such yogurt brands.

There is an option is to use β-galactosidase to hydrolyse lactose as the

products are glucose and galactose, which are much sweeter than lactose.

Good Luck.

Galactosidase or lactase does help, makes a reduction of app. 2% sugar possible, but not 10%. Addition of artificular or natural sweetener helps, but dosn't give the mouthfeel of sugar, so a combination of a milder culture, lactase, maybe Stevia and a smaller amount of sugar is good for a all natural product, and aspartame is good if that can be considered.

hi...replace some added sugar with Oligosaccharides (FOS).

Add low calorie natural sweetener like those from stevia