If using sour cherries, bump the sugar by two tablespoons and lower the amount of lemon juice to two tablespoons. This will account for the natural tartness of sour cherries and balance it out. This sour cherry pie filling will keep in the fridge for several days without a problem, but it will thicken. Remove it from the fridge an hour or more before using. Whatever you do, DO NOT microwave. Microwaving will ruin its velvety smooth texture and cause it to become grainy.