"Owner wants to bring in a partner with ownership rights"

New entity, new FEIN.

Or, they can form an S Corp directly. Or a C Corp.

Remember that LLC (multi-member partnership or entity) is a State issue. That means applicable State legal guidance should be sought.

Being any sort of corporation or staying as an LLC (or PLLC or whatever might apply in their State for their activity) should be done with an understanding of the intended operations. We've seen lots of bad guidance on this topic.