Thank you for the exhaustive replies!
I do know why the tax return was amended, probably I should have put that in the first post.
I have dependents with ITINs that apparently had expired. Therefore those individuals were discounted from my return, which made a big difference. I can see the amendments on my tax transcript.
I don't know if it's too late to get ITINs and contest the adjustment. I was trying to get advice on this question but first I wanted to get a copy of the notice.
