I currently work on an H1B for a company A. I plan to switch (through an H1 Transfer) to company B. My original visa was issued in 2008, and I plan to transfer for another 3 year period.

I have question regarding start date and notice to the current employer. Should I inform the present company once I get a receipt notice from USCIS and join the new company immediately OR should I wait until I receive H1 approval notice.

If I can switch after receiving the receipt notice, do you suggest I use premium processing if I'm targeting April 4th join date?

Thanks,
RN