Re-entering USA to activate new employer's i797

[mrawal]

I currently work for employer A but have an approved H1B transfer for new employer B. I am currently out of country and would need to work for employer A for a week after coming back before joining employer B.

Employer B's lawyers are asking me to go outside US after completing the remaining week under employer A and re-enter the country with employer B's i797 to activate the new i797. Is this necessary?