Remember that if you increase the ABV, and therefore the OG of the beer that you must also increase the hops to balance the beer. Unless you're adept with adjusting recipes, it's probably better to start with the right recipe from the start. For example, if you want a 6% ABV oatmeal stout, start with a recipe for a 6% oatmeal stout. It's really important to balance the bittering hops with the amount of malt used, so that you don't end up with a too-sweet beer.