Q. If a recipe calls for packed brown sugar, can I substitute granulated brown sugar and if so how would I measure granulated brown sugar to equal , say, a half cup of packed brown sugar?
A. You’ll have to use your judgment, but we would say don’t. The folks at Domino Sugar say you can substitute their "Brownulated" light brown sugar measure for measure with regular brown sugar in recipes, but and it’s a huge but because the moisture content is different it will produce "differences in texture as compared to recipes made with regular brown sugar."
If that unknown isn’t enough of a deterrent, Sharon Tyler Herbst, author of The
New Food Lover’s Companion (Canada, UK) flat out says don't do it.