The bad news is that at the top of the page there is a statement: "Juicebox is currently on version 1.4.4.2";
Oops! Looks like that one slipped through the net. Now changed to "v1.5.0". Thank you for pointing it out.
... with the release of 1.5, is copying its jbcore directory to the web host sufficient to update existing galleries?
Yes (as long as you were previously using v1.4.4.2).
The change between v1.4.4.1 and v1.4.4.2 was to the 'index.html' files (generated by JuiceboxBuilder-Pro) to ensure that the correct meta viewport tag was included in the gallery web pages for iOS 9 support.
All the changes from v1.4.4.2 to v1.5.0 are in the 'jbcore' folder so swapping your 'jbcore' folders is all you need to do to upgrade your existing galleries.
If you are upgrading from v1.4.4.1 (or earlier) and are using the 'index.html' files generated by JuiceboxBuilder-Pro, I would recommend rebuilding your galleries with JuiceboxBuilder-Pro v1.5.0 (to upgrade both the 'index.html' files and 'jbcore' folders).
Otherwise, you could manually swap your 'jbcore' folders and ensure that the meta viewport tag (noted in this Juicebox blog entry) is in the head section of your gallery web pages.