UPGRADE TO BIGSUR

This will all depending on what version is presently installed. Like any upGrade to a new OS - due diligence is a requirement. Check with Apps Developers if they offer updates / upgrades for Big Sur. Same goes for Drivers for Printers, scanners, etc.

Make a Time Machine Backup Before venturing to Big Sur and while at it - make two Tested Bootable Clones each to separate external drives - added insurance incase Big Sur goes sideways or not to your liking.

No. It will be faster.

If time allows - do keep us updated on progress.