401k Roth account to Roth IRA -- do this directly, as a separate transaction. No tax implications.
Pre-tax 401k to Roth IRA will be a taxable conversion. The tax consequences of this are challenging to explain. You could do it in one step, direct from the 401k to the Roth IRA, This would be a conversion that is fully taxable and you would owe about $12,000 of federal income tax, plus state (depending on your other income, of course). Or you could do it in two steps, from the 401k to a traditional IRA, then from the traditional IRA to a Roth IRA. You could move the entire amount from the 401k to a traditional IRA tax-free, then only rollover (convert) from the traditional IRA to the Roth IRA, as much as you could afford to pay in taxes at the time. However, if you have pre-tax funds in a traditional IRA, you can't do a backdoor Roth contribution with other money.
For $55,000 conversion, you would expect to pay $12,000 of income tax. Do you have $12,000 of other money to pay the tax? If not, then don't transfer more in any one year than you can pay the tax on. For example, if you have $2200 available for taxes, do a direct transfer of $10,000 from the pre-tax 401k to the Roth IRA, then pay $2200 to the IRS as an estimated tax payment (www.irs.gov/payments). Then next year, perform another transfer in whatever amount you can afford to pay the taxes on.
If you do an intermediate transfer of the entire pre-tax 401k to a traditional IRA but can't afford to convert it all to a Roth IRA this year, this will prevent you from doing a true "backdoor Roth IRA" contribution, because you will have a pretax amount in the traditional IRA which means you must use the pro-rata rule on any conversion.
On the other hand, if you can afford to pay the entire tax on the Roth conversion from other money, then it doesn't matter if you convert the 401k in one step or two. If you rollover $55,000 of pre-tax into the traditional IRA, then contribute $7000 after-tax, and do the Roth conversion all at once, you will only pay tax on the $55,000 (which is the same amount of tax you would pay if you did a direct rollover from the pre-tax 401k to the Roth IRA).
As a separate issue, if you do the conversion all at once and owe a tax payment of around $12,000, you may run into issues with penalties for not having enough tax withheld during the year, even if you make the estimated payments on time. There are some ways to deal with this, I won't go into it now. But if you want to reply and tell us what kind of schedule you plan to use to convert the pre-tax 401k, we can discuss the payment issues separately.
Lastly, you should look at the investment options. Employer plans sometimes offer limited funds, which limits your investment options. But the employer can usually negotiate lower fees and expense ratios. For example, I own the same fund in a Roth IRA and a 403b account, and the fund has a 0.2% expense ratio in the employer plan and a 0.9% expense ratio in the private plan. That could be significant over time. It might be worth leaving your money in the employer plan if those lower fees and expenses mean you get a higher net rate of return.
