You cannot legally begin work for the new employer until USCIS issues the receipt notice confirming the transfer petition. Working without this can be considered unauthorized employment and may jeopardize your status. If you are maintaining valid H-1B employment with your current employer, you must continue working there until the receipt notice is issued. Keep all documentation to demonstrate compliance in case of future inquiries.

Once the receipt notice is issued, you may start work under portability rules, assuming you are in valid status. If there are delays in receiving the notice, it is important to maintain your current employment or other lawful status. Keep detailed records of all filings and communications with USCIS, as this documentation can protect you if any questions arise about your eligibility to start employment with the new sponsor.