After the year the you do the rollover to the 401(k), converting when you have a traditional IRA value of less than your basis can result in unrecoverable basis, so either convert immediately and avoid investment losses in the traditional IRA or retain a small balance in your traditional IRA by doing a partial Roth conversion to preserve your basis for application to future Roth conversions, perhaps eventually by rolling some money from the 401(k) to the traditional IRA and then converting to Roth.
Unrecoverable basis just means nondeductible contributions that you put in that you'll never be able to get back out because it went to investment losses. In the past you could potentially take a miscellaneous deduction for unrecoverable basis (which presumably used up the basis) if you had a zero balance in all of your traditional IRAs at year end, but that deduction was eliminated by the Tax Cuts and Jobs act of 2017 and the IRS has not made it clear what happens to your unrecoverable basis now when your year-end balance in traditional IRAs drops to zero. Maintaining a small balance in a traditional IRA ensures that the basis continues to be carried forward for potential later use rather than just possibly disappearing and producing no benefit. (My own position on this question is that the basis should continue to carry forward even with a zero balance in traditional IRAs, but I've never seen a ruling to confirm this.)