change from H1B to O1 visa

[EmBlakey]

My husband was just granted an O1 visa and changed it from H1B. They informed us that the visa has *already* been granted and the H1B is already invalid. He is still working for the original employer as he has to give 2 months notice to leave. The new employment doesn't start until January. How does this work legally with regards to the old visa being invalid but still working for that employer? He couldn't give notice until he knew he visa would be approved..
Thanks!