Can I amend if I already received my refund?
Of course. In fact, the government doesn't want you to amend until you've already gotten your tax refund.
You're free to cash your refund check or spend it once you have it. You don't need to wait for your amendment to finish processing, which can take another 3-4 months.
If you owe money after amending, you'll just include payment with your mailed amendment form.
And if your amendment results in a second refund, you'll get a check for the additional amount.