1 - Will changing to a Roth 401k affect the abovementioned Roth IRA conversions in any way ? Is there any tax I should beware of? No ... none.
2 - What is the best way to Rollover the current 401k balance (less than $10k) to a Roth 401k ? Will that affect the Roth IRA contributions in any way? Ask the employer if it can be done ... doubtful it can. You will simply have a balance in each 401K account going forward.
If you are allowed to roll the 401K to an IRA and then convert it to a ROTH then go that route. FYI all the balance in the IRA at the end of the year must all be converted to keep this "back door" system you have going clean.
