You must first UNDO all the changes that you made, and put the original return back to exactly what it was when you filed it. When you amend, the 1040-X asks about changes you wish to make, and prepares a completely separate form for those. And yes, you will have to explain the changes. Here are some examples:
o Forgot to claim a late arriving W-2.
o Added overlooked medical expenses.
o Corrected mistyped 1099-R.
o Deleted a dependent.
