Recharacterization of 401(k) elective deferrals is not permitted. However, if your plan permits, you can do an In-plan Roth Rollover of some or all of your balance in the traditional 401(k) account to the designated Roth account in the same 401(k) plan. The original elective deferrals you made for the year will still be excluded from box 1 of your W-2 but you'll get a code-G Form 1099-R reporting the gross and taxable amounts of the IRR. The taxable amount will be the same as the gross amount unless you have after-tax funds in the traditional 401(k) account which would lower the taxable amount. The taxable amount will add to your AGI. To be 2022 income, the IRR has to be performed before the end of 2022.
Be aware that although Roth conversions from a traditional IRA are subtracted from AGI when determining your MAGI for the purpose of making a Roth IRA contribution, the taxable amount of an IRR is not subtracted. (Of course this only matters if you make a Roth IRA contribution for the year.)
