How can I capture the previous version of an edited comment using Jira Automation?

Just to close the loop on the automation side, since this comes up a lot: {{changelog.comment.fromString}} is empty by design, not by bug. Jira never stores the previous text of a comment, it overwrites it, so there is nothing in the changelog for the smart value to read. No amount of automation, ScriptRunner or REST access recovers it, because it was never persisted. The only thing that works is capturing the text at the moment of the change, going forward.

The custom-field approach Ariel suggested is the honest free option and worth knowing. Two limits to plan around: it records the new text on each edit, so you get a running log rather than a before/after pair, and anyone who can edit fields can edit the log, which matters if the reason you want this is auditability.

On apps, Charlie's two are the main ones. Disclosure: there is now a third and I make it, Comment Audit Trail for Jira. All three share the same limit, history starts the day you install, so whichever you pick, installing sooner is worth more than picking perfectly.

Whatever you choose, two things worth checking on the listing first: how many permissions it requests and how many of those grant write access (for an audit trail it matters, because a record that can be altered proves less), and whether your comment text stays inside Atlassian or is processed on vendor servers. Both are published before you install.