You do not report this as a change on your business income tax return. Instead, you'll just file a s-corporation return this year, form 1120-S. When starting a return, you'll be asked about the type of form.
To make sure you've completed everything needed to qualify, be sure to check out the steps at the IRS for making this change: https://www.irs.gov/businesses/corporations/filing-requirements-for-filing-status-change-revised-11-...
