Hi,
You need to upgrade to an intermediate version of the software before you can upgrade to 8.20. Acceptable versions are 7.30.4.02, 7.40.2.02 and 7.50.1.03. This is documented in Appendix A of the CSS 8.20 Admin manual - as is the upgrade procedure. AFAIK this is the only document that describes the update process.
For the upgrade you can use either the .adi or .adi.gz file - the CSS will automatically decompress the gzip version if you choose to use it.
Check the Release Notes for any changes to syntax for commands that you use.
I've used the automatic update process - assuming that the script would know what to do. I did make sure that I had backups of the config archived on the CSS and on the FTP server - along with certificates.
HTH
Cathy