Can I switch my LLC from sole-proprietor taxation to an S-corp mid-year?

Yes, but timing rules matter. To have an S-corp election apply for the whole tax year, the IRS generally wants Form 2553 filed within two months and 15 days of the start of that year. Miss that window and you can request late-election relief under Rev. Proc. 2013-30 if you have reasonable cause, or the election simply takes effect the following year. Some owners intentionally elect mid-year once profit is high enough to justify payroll. Remember an S-corp requires running actual payroll for your reasonable salary, filing Form 1120-S, and issuing yourself a W-2, so the administrative cost only pays off above roughly $40,000 to $80,000 in net profit. Model the breakeven before you file, and confirm your state recognizes the federal election.