Be Careful If You Own an Old Mac
macOS Big Sur is available for Macs as old as 2013, which is great for long term support, but if you are at the edge of this compatibility, you may want to hold off on installing the update. Let other users try out macOS Big Sur on an old Mac to see how it performs.
This will let you decide if the performance is fine, or if there are issues that will prevent you from reliably using your Mac. Check sites like the MacRumors forums and Apple Discussion forums to hear from users with similar devices.
The update is apparently bricking older MacBook models. The problems appear fairly widespread and they're mostly impacting owners of 2013 MacBook Pro and mid-2014 MacBook Pro models. These are the oldest MacBook models supported by macOS Big Sur.
You're able to downgrade to macOS Catalina so you can test macOS Big Sur out, but you'll want to make sure you have a good backup and that you have time to downgrade if macOS Big Sur isn't what you expected.