How about adding a rule that increases the rating one level up? You can add this rule on the checkbox itself but also add a condition to that rule that checks if the checkbox value is not true. You also need to add a condition on your existing rule so that it runs only when checkbox = true.
So, in your checkbox field, you'll have two rules with conditions:
- If checkbox = true, lower the rating one level
- If checkbox != true, increase the rating one level
Hope it helps!