You are correct that most of these programs default to the date of the contribution matching that tax year. This is certainly an error prone situation if the employee does not monitor things. Rather than re classifying contributions already made there would be less risk of error to determine how much in 2018 contributions have already been coded by the IRA custodian, and if there is room for more, make that contribution separately and leave the already contributed amounts as is. That said, any method of changing the actual 2018 contribution amount that will be reported on Form 5498 can be reported on Form 8880 as a contribution for the applicable year. Form 8880 and the 5498 must be in sync unless a contribution is later removed or recharacterized.