I pretty much use old fashion/reg. oatmeal in place of quick in baking all the time, also. Haven't seen much difference. It's healthier (can we say that when talking about cookies???) .
I use them interchangeably. They yield slightly different textures...
I often mix them. My oatmeal cookie recipe calls for 4 cups of old fashioned oats but I sub in 1C of quick oats because I like the texture better. I have tried this in several different cookie recipes with great success!