User blog comment:IamJakuhoRaikoben/Chapter 474: In the Moment of Complete Silence, Review./@comment-121.54.44.93-20160224061001/@comment-7418318-20160224091225

Wouldn't work.

Retrogression is a curse that reduces people's abilities and magic. That their bodies' appearance goes to a younger age is just a side-effect. If Ultear really had been affected by Retrogression, she's be unable to use anything but the most basic of magics, certainly nothing on the scale to coutner one of the 12 Shields.

Besides, it's a curse that needs to be maintained by the caster. The moment Doriate doesn't actively keep the curse going, it goes out.