If the rollover from the 401(k) to the traditional IRA is done by direct rollover, tax withholding is not required. Tax withholding on the Roth conversion can also be declined. With no tax withholding, you might have to make timely estimated tax payments to avoid an underpayment penalty.
If your intent is to end up with some or all of money in a Roth IRA, it's generally better to do a taxable rollover of that portion directly from the traditional 401(k) to the Roth IRA, particularly if there is any after tax basis in the traditional 401(k) account and you already have other money in traditional IRAs. The 401(k) plan is required by law to permit a rollover directly to a Roth IRA.
The 401(k) plan is required by law to provide you with all of this rollover information between 30 and 180 days prior to making the distribution unless you waive the 30-day period.
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