New I-94 form vs. Change of employer status

Could someone please explain to me the difference between applying for a new TN visa/I-94 form and petitioning for a change of employer?

Thank you.

Getting a new TN at the border means applying for TN status at the border and CBP issues an I-94 card. A change of employer is a TN petition filed with USCIS. You don't depart the U.S. during this process. You have to maintain employment with the first TN employer to be able to file with the second employer, and there is no H-1B portability: you cannot begin working for the new TN employer until the petition is approved. Unless you premium process, it will take months for USCIS approval.