Can I substitute buttermilk for sour cream in a coffee cake recipe if I increase the amount of flour?

I"ve substituted buttermilk (homemade, lemon juice + milk) and also kefir for sour cream in coffeecake recipes with excellent results.

By doing so you will get a drier, more open crumb. If that textural change doesn't bother you, I agree with the others, there's no need to add additional flour.

I agree with kmack, you don't need to increase the flour. If the volume of buttermilk is identical to that of the sour cream, you also don't need to change the baking soda. Otherwise, the proportion should be 1/2 tsp. baking soda to 8 oz. buttermilk.

You can, it helps if you add butter (3/4 C buttermilk 1/3 C butter), I'm not sure if an increase in flour is necessary. I've also substituted sour milk before with decent results.