OPT extension with a new STEM code - same curriculum

My program recently received the STEM category code. Its the same curriculum with just a new code and new I20. My opt expires mid april and i intend to apply for H1 by april 1 to be in legal status. I am filling for an OPT extension under the new code on the I20 - will new EAD if issued or the application receipt allow me a longer OPT time period so that i can apply for an H1 from a new employer little later. Once an H1 is applied from current company can i transfer to another company while i am in the OPT extension period and file a new H1 through another company. I beleive once cap-gap period starts one cannot change the employers but its possible to do so on an OPT extension.

My new I20 is under the STEM category now but it was issued recently - after my OPT started. ITs the same curciculm just a diffferent code approved by USCIS.

My univ is allowing for OPT extension applcn as the new code is for the same curriculum but all my past I20s have been issued for a different code.. Is there any chance of rejection for an extension