Once you have a basis, as you said, it stays with the IRA(s) until you liquidate all IRAs.
Meanwhile Form 8606 computes the non-taxable part of any distribution. your basis goes down but only fractionally.
File a 2017 8606 with the IRS showing your correct basis.
OR,
abandon your basis. you will be taxed twice on that amount as time goes on, but maybe that doesn't matter to you if 8606 is a hassle.
