The Release Notes for each of the modules (and their upgrades) generally describe what has changed and things to look out for.

However, there are enormous changes between AR 8.0B and 9.0A, and Ledger 8.0D and 9.0C, so I wouldn't recommend trying to do these upgrades on your own.

For a taste of the changes, you could copy your data to a local drive, install the upgrades locally and convert the data, then make sure that everything is still working the way you expect (especially forms such as statements and invoices).

You have a bit of reading ahead of you.