More simply, the tax code does not permit changing a Roth 401(k) contribution intentionally made by the employee to be a traditional contribution instead, as is the case here.
The only possible way I can see for it to be permissible for a contribution deposited into to a Roth 401(k) account to be moved to a traditional account is if the election provided to the employer by the employee was to deposit the contribution into the traditional 401(k) account but the plan administrator mistakenly deposited it into the Roth 401(k). Correcting this error on the part of the plan administrator would not be a recharacterization but would instead be a correction of a bookkeeping error.
