H1B transfer

I got an offer from another company, I signed the offer and that company just finished the H1B premium transfer( got the receipt). That company asked me when I can start to work for them

However, the job deion(bachelor + 4 years or MS + 2 years) may turn me to EB3 green card type.

My question: Could I reject to join that company? My current company will do the EB2 green card for me.

could anyone who has such experience or knowledge know this?
Thanks for your time!