Upgrading from 4.26.8 to latest revision?

[Brian Lewis]

What is the correct procedure to safely upgrade from 4.26.8 to the latest
revision?  The Installation Documentation and FAQ don't seem to mention
an 'upgrade' procedure.

Do I still use the ./install.sh script? (or is there an upgrade.sh script
somewhere??)
Do I need to backup any configuration files in the /etc/MailScanner folder?
Which files are commonly modified during this install that I should be
concerned it?

MailScanner 4.26.8 currently
SpamAssassin
ClamAV (/usr/lib/MailScanner/clamav-wrapper modified to use tmpfs
partition)
512mb TMPFS partition for clamav and mailscanner
/etc/MailScanner/spam.assassin.prefs.conf modified
/etc/MailScanner/filename.rules.conf modified
/etc/MailScanner/MailScanner.conf modified