A corporation filed their 2019 taxes in February 2020 on form 1120 as C-corp.  They must have also filed an election to be treated as an S-corp because in Sept 2020 they received the IRS letter stating their election to be treated as an S-corp effective Jan 1, 2019 was approved.

Since their s-corp election was effective Jan 1, 2019, shouldn't the 2019 tax return have been filed on an 1120-S?   Can they leave 2019 filed on a regular 1120, or do they need to go back and amend to an 1120-S?

Thanks in advance!