2021 is a preview/beta version, best run on another computer or another account on the same computer, not on your main computer with your real data set. Not seen any run for the hills data corrupting issues, but at your own risk.
This is my understanding.
If you have a 2019/2020 key it will work with 2021. If it’s 2017 key or earlier it want work with 2021.
At the moment the only way to get a 2021 key is as below. A 2021 key will give you the next major upgrade free and after that at a 50% discount.
Until 2021 is a released version, any keys from this link will not work.
In Moneydance 2021:
If you are a new user that does not have a key click PURCHASE.
If you are an existing user whose key is valid for 2021 just carry on
If you are an existing user whose key was for 2017 or earlier and thus is not valid for 2021 click UPGRADE to purchase a new key at a 50% discount.
Note a 2021 key will not work with 2020 or earlier versions.