Depending on what version you had before, this maybe a free update, or failing that you should be entitled to 50% discount. You can check it out on the purchase page see upgrading at :- https://infinitekind.com/purchase prior to downloading the latest version.
Your MoneyDance Data should have been transferred with Migration Assistant & the new install should find & update your data.
If I might add any Moneydance release prior to 2021.1 is probably going to have some issues with Big Sur, issues around later versions of macOS not allowing existing owned versions of applications to work properly have been around for some years.
If you need to manually move your data the following Knowledge Base article might help: