License valid period

The 2020 license has been purchased and used, and the 2021 license has also been purchased.
From when is the license effective period in 2020?
Is the 2020 license only valid until 12/31?
My system is going to patch on 1/12/2021.
At that time, I plan to change the key to the purchase license in 2021
Then, is an error appearing from 2021/1/1 to 2021/1/12?
Please inquire about any side effects.
I will use the license key for 2020 until 1/12/2021, and after that, I want to replace it with the 2021 license key.
Then, we ask if there will be any problems with use from 1/1 to 1/12.