Changed from sole proprietor to llc mid year, how do i file

Are you the only LLC member?

For federal income tax purposes, a single member LLC is a disregarded entity.  As such, nothing really changed for federal income tax purposes.  Both will be reported on a Schedule C.

How about if they elect to be taxed as an SCorp. Do they file as sole pro for mid year and then 1120S? Or do they file 1120S for entire year?

Correct, they would do a partial sch c, then file an S corp with a short fiscal year. It determines what is appropriately taxed for SE. Incorporating mid year then lumping all on s corp is akin to not taking reasonable compensation. Dont forget partial year w-2 from s corp.

Cheers