It's not necessarily so simple. For example, the 1099-R increases your income. The additional income could change a deduction or credit on another form. As another example, if the 1099-R that you added is an early withdrawal from an IRA, you would need Form 5329. These are just a couple of examples. There are other situations where a form could be added or changed.
I don't have a simple answer for you. You should compare the entire amended return to the original return to see what forms have changed or been added.
