Prepare an amended return for each year, but only file the return(s) that show a balance due. This is the most accurate way to see whether or not it is necessary to amend the return.
Pay any balance due when you file your amended return.
For any balance due, you will also receive a bill for penalties and interest from the IRS at a later date. It can take many months for the IRS to process an amended return, so it may be a while before you see that bill. The refunds from Canada are yours to keep, you don't send them to the IRS.
You can learn more about filing amended returns here: How To File an Amended Tax Return.
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