Got approvel of COS with follow mail for I797 mail from USCIS

Hi ,

I got L1 to H1 COS approval notice from mail after submitting RFE 3 days before. Current L1 I94 is valid till Jan 2015. Should i resign or need to wait for mail of I797 or Hard copy from USCIS.Is it leagal to join on old L1B I94 if in worst case new H1B is not comming with new I94 or need to do stamping again.

Please reply asap. Thanks for helping so many people always.