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.
