During a code review, is it possible for the author to replace their initial dev branch in the code review with a second dev branch? I have noticed that if a changelist from a second dev is added to the code review, the first dev branch remains in the code review, but I would like it to be overwritten if the same files are changed.
I'm not sure what do you mean by first and second "dev branch", but I'll suppose you're referring to the first and second "patchsets".
You can't delete patchsets (only the entire change), but you don't need to worry about the previous patchset because at the end just the latest patchset will be merged to the branch (Git repository) and all previous patchsets will remain available just in the change (Gerrit interface) for historical purposes.