Decouple mixed use properties

[K S.]

I wanted to make an offer on a mixed 1000 sqft mixed use property in Southern Cali. The property has a residential unit upstairs and a commercial unit downstairs.

They both have their own mailbox address, front and rear entrances, bathrooms etc. The only thing that makes them coupled I think is the spiral staircase connecting the two. However, even the spiral staircase is in a private foyer between behind closed doors on each unit meaning it can either be removed, blocked or just lock the door going to the foyer.

So how hard would this be to rezone and decouple the commercial unit with the residential? This way I can refinance the residential side into a conventional mortgage. I don't understand the dual use from an owners perspective. Most of the commercial tenants don't live in the residential units anyways so I'm having a hard time understanding the benefits.