The question is not about changing the contributions to Roth. The question is whether they can be changed to nondeductible traditional IRA contributions. If the deduction was actually present on the original tax returns, it's unlikely that they could now be changed after 30 years. Such filings would not have been erroneous, just not beneficial. If the deduction had not been present as an adjustment to income on Form 1040, you might be able to file the late Forms 8606 if you can show reasonable cause, but there would likely be a $50 late-filing penalty for each.
