First, you have until the businesses tax filing deadline including extensions to remove excess SEP IRA contributions. Second, you may not have to remove the excess contributions. Call Schwab and explain the problem. Their SEP IRA is a prototype SEP IRA which can be maintained with a 401k for the same tax year.
I believe (with verification from Schwab), you can adopt a prototype SEP IRA, rollover the 5305-SEP IRA balance to the Schwab prototype SEP IRA, adopt a one-participant 401k and make the employee deferrals. Then you can decide if you want to rollover the SEP IRA to the one-participant 401k and even whether to make future employer contributions to the SEP IRA or 401k (subject to the annual addition limit either way).
Note: The sooner your one-participant 401k year-end balance reaches $250K, the sooner you will have to file Form 5500-EZ. However, there maybe other valid reasons to rollover the SEP IRA balance and make future employer contributions to the one-participant 401k.