I am currently on H1-B.
I recently switched employer and have been working with the new employer for 1 month.

I would like to go back and work for my previous employer.
My previous employer is ready to hire my back and has also decided to hold my H1-B. He has not cancelled my H1-B.

The visa on my passport and I-94 specify the WAC# of the previous employer.

Can I go back and work for my previous employer right away or do I need to step out of the country and get back in with the new and old I-797 petition?

Kindly advise.
Thank You