I noticed there's a higher resolution version of post #998169 available at https://www.furaffinity.net/full/17558982/, but there's one minor difference: the fur color on the FA link's image is slightly brighter.

Does a very minor difference like that in an otherwise identical image warrant being marked as an edit, or is it worth flagging the one currently here? I ask for feedback since I don't wish to flag older posts without things being identical, typically.

+1 for replacement. I see this sometimes, with the commissioner posting a better version than the artist. The shading difference is minor and does not warrant a separate post in my opinion. You should link the alternative version in the source field, and you could add the alternate_version_available tag.

That's usually fine, unless the original was posted here by the artist. Those we tend to keep, since it's up to the artist to decide which version they want here.