If I review an in-progress pull request and subsequent changes are necessary, I have three options.

First, I can review the entire pull request again from scratch. Reviews take a great deal of time and having to start from scratch, reviewing everything again is a pain. In absence of better options, I often do this, limiting my number of reviews because it's so time consuming.

The second option is to review the commits individually as they've happened since your previous review. From here, you can make commit comments (which have an odd way of showing up in the UI, but they work) on the individual commits. The downside to this is you're subjected to the stream of consciousness of the developer committing code. If they're the sort of person who makes lots of smaller commits, you can get lost in the small changes in a localized area of the code without getting a sense of how it fits in the larger change.

The last option is url hacking. Looking at the commit SHA where you made your review and the current SHA, you can diff them with a url like the following: https://github.com/django/django/compare/62b74ea...31e29a2 where the two SHAs in the URL are the range you want to compare (exclusive of the origin). This is pretty helpful in condensing the issues with the individual commit review option, but unfortunately it takes you out of reviewing mode. This means that you may see an issue in the code, but you can't comment on it here. You have to either go to the commit in which it was introduced or find it in the full diff of the main PR.