H1B transfer and GC refilling

[RManickam]

Hello Sir, My H1B visa expired on Sep 2019 and now 5th year running. (I-140 approved).  Now i got an offer from company A, they said after one year they will start GC process. My question is if they not started after a year what will happen? 2. After join company A can change the employer again before new H1B approved? 

Thanks