Off Calendar Safe Harbor Startup

So a last minute client would like to set up a safe harbor 3% in order to make deferrals for 2020.  

If we set up a plan with a plan year end of 11/30/2021, when can it be amended to a calendar year plan.

Can it be amended 3/1/2021 for a short plan year to 12/31/2021, thus providing for a 3 month initial plan year.

Or does the first plan year need to run for 12 months before a short plan year can be created, meaning a short plan year starting 12/1/2021 and ending 12/31/2021, and the calendar year plan starting 1/1/2022?

Thank you.

I believe the first plan year will have to be 12 months long.  Can anyone confirm?