I did this recently. “Post-tax contributions” go into a 401a account rather than your existing 401k. So before you call Fidelity to direct them to convert the contributions, first you have to set up 401a contribution withholdings from your paycheck. You can set the 401a post-tax retirement contributions up on the same election page where you specify the percentage of withholding for your 401k.

(It is a little confusing at first because on your Fidelity portfolio page, there is only a single line item for Delta retirement money so it’s hard to tell initially that you’ve established a new withholding. But if you drill down on the site, there is a link for “sources” where it breaks out 401k, catchup contributions if you make those, Roth 401k, and 401a separately.)

Once there is money going into the 401a, then you call Fidelity and tell them you want an “in plan conversion” so that the 401a money is treated like a Roth account, the benefit being that while you have been taxed on the amount you contributed, you don’t pay tax on the earnings on the contributed funds. “In plan conversion” are the magic words. Also note there is a special 401a group at Fidelity to whom you’ll be transferred when you call.

If you don’t call for the in plan conversion, effectively all you have with the 401a is a taxable brokerage account that is funded with automatic paycheck withdrawals, under the umbrella of the retirement plan.

The ALPA volunteers who assist with retirement issues can help. When I submitted a DART they got back with me quickly and were great.