SEP IRA rollover to a 401K

Business is an existing SEP IRA Plan. The business wants to set-up a 401k plan for 2023. Can the employee's rollover the SEP into the 401k in 2023? The employer is not a new employer, and the 401k plan administrators will allow a roll-over.

Sure, why not? He should be able to roll over any non-Roth IRA into a 401(k) (although there is a 2-year waiting period on SIMPLE IRA amounts).