1. Yes.
2. The prorating is based on your 2023 year-end traditional IRA balance. Your year-end traditional IRA balance will be reduced by whatever amount of your pre-tax funds you roll over to the new 401(k) before year-end. The rollover to the 401(k) must come entirely from pre-tax funds in the traditional IRA (including, perhaps any earnings in the traditional IRA). After-tax funds are not permitted to be rolled over to the 401(k) (nor would you have reason to do so). Prorating is still done, but with a year-end traditional IRA balance of zero the calculation results in all of your basis being applied to the Roth conversion (unless the amount converted is less than your basis in nondeductible traditional IRA contributions due to any net investment loss that occurs after the rollover to the 401(k)).
