Oblivious Investor
says conversion. Specifically, it's treated as a 401(k) and has the same rules (qualified, non-qualified distributions, 10% penalty plus taxes on earnings if certain criteria aren't met).
The 401(K)-ness means you have to play by your 401(k)'s rules, which may or may not allow for in-service distributions.
Importantly, when you withdraw (assuming you can), while you don't pay taxes on contributions, you don't get the Roth IRA advantage of withdrawing contributions first, then earnings. Instead, it all comes out pro-rata: if you have $10,000 earnings in your $100,000 Roth 401(k), 10% of any distributions will be earnings.