"The IRS does not offer a standard form for changing your company’s tax status from S corporation to C corporation. Instead, it simply requires a written statement be filed with the appropriate IRS service center, along with a consent signed by a majority (more than 50%) of your corporation’s shareholders .

The written statement can specify a date as long as it is not prior to the filing of the actual tax status reversal. If no specific date is listed, the filing becomes effective during the current tax year as long as the revocation is filed by the fifteenth (15th) day of the third month of the current tax year. If it is filed after that date, it becomes effective as of the first day of the following tax year."

At least that's what they say here.

https://www.delawareinc.com/blog/why-convert-your-tax-status-from-s-corp-to-c-corp/

I would think it might depend on whether the change was voluntary or mandatory; when it was requested; and whether the request for the change specified a date.