The downsizing of cake mixes was discussed in this thread some time back. It was suggested you add flour to compensate. I just came across this mix for up-sizing, in case it can be of help to someone.
Haven't tried it yet, but have high hopes for making SheRa's Margarita Cake again.
1 1/2 cups AP flour
1 cup sugar
2 teaspoons baking powder
1/4 teaspoon baking soda
Whisk all dry ingredients together and store in a clean jar.
When you wish to increase a 15.25-ounce cake mix to 18.25 ounces, add 3 ounces of this mix (6 tablespoons) to your cake mix and bake according to directions.
ETA: Using sugar in the "upsizer" makes me think of Beth's Sugar Creep thread
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