I would obviously change a post date manually if I added more than 300 words to a blog post. But is it okay (or not) to change the date of the post if the tweaks are 20 words and/or photos? I worry that to Google I will seem to be saying: "See how great my post is - update, update, rank me higher, YES I KNOW it was a small change but it is oh-so-important". Sure logic would imply that authority, niche, EXPANDING content is key to good SEO. But does "expanding" allow for constant re-dating or is it best to simply make a new page (page II of I)? Thoughts on often re-dating of a blog post welcome.
I think a publication date should never¹ change. It’s, as the name suggests, the date of the publication, not the date of the last change to the published content.
If you want to indicate that you edited the content, provide a (last) modification date.
This difference can be conveyed with structured data: Schema.org offers the properties
Google Search recognizes this difference, for example, in their Articles search result feature.
¹ Well, one case where it should be changed is if the original content gets deleted and the page gets new/unrelated content (e.g., after the site owner changed or whatever). But in such a case the URL of the page should change, too, ideally.
It is normal for bloggers to update existing posts with new information. If you have set the permalink of your blog to reflect the date of the post, then changing the date could change the URL of the post as well. If you have existing links pointing to the URL, then you might notice a temporary drop in the ranks of that page till the time the link equity is 301 redirected to the new URL
However, if changing the date of the post doesn't change the URL, then I don't think Google will have any issue with the modification, because technically you're updating the same page and not creating a new one.