Move a traditional 401k balance to a traditional IRA (not Roth IRA) in order to avoid a taxable event. Converting traditional funds to Roth can be done at anytime but would be a taxable event so best to convert during low earning years (12% federal tax bracket while living in a state with 0% income tax) like the years after early retirement but prior to collecting SS at age 70.
Advantages of leaving money in a 401k instead of moving to an IRA include (1) enable rule of 55 withdrawals, (2) better protection from judgements, and (3) enabling backdoor Roth IRA contributions without pro-rata tax payments.