Whatever your decree may say doesn't matter to the IRS. The IRS has their own set of rules for divorced parents, as to who can claim the child. Only a federal judge can change that. Since federal judges don't deal with separation, divorce or custody cases, I'm highly confident that will never happen.
Under the IRS rules, it's perfectly possible for who the IRS recognizes as the custodial parent to change year to year. Only the custodial parent as defined by the IRS can claim HOH, provided of course they meet the other requirements for that filing status. This is true even if the custodial parent signs IRS Form 8832 giving the non-custodial parent the right to claim the child for that year.
