You can’t do that, once the topic or a specific reply has been bookmarked the button at the topic level changes to ‘clear bookmarks’. The topic-level control to bookmark is gone.
Bookmark set at topic level
Bookmark your topic instead
So a topic-level bookmark can’t overwrite a post-level bookmark, if a specific post is bookmarked the post and latest reading position are the two links within the bookmark. If the topic is bookmarked then it’s the topic root / and the latest reading position.
I bookmarked your earlier reply, so my bookmark link goes to /4 and my topic link goes to /6 because it’s my latest read position.