"How do I replace a userpic? What happens when I change my default?"
Userpics are assigned to entries and comments through their keywords. If you post an entry and select the userpic with the keyword "I love LJ", that entry will display the userpic associated with that keyword.
If you want to replace an older version of a userpic with a new version, or a different userpic meaning the same thing, remove the keyword from the old userpic, upload the new userpic, and give the keyword to the new userpic. All entries and comments using this userpic keyword will automatically display the new userpic.
There is no time limit for when you may upload a new userpic with the same keyword. As long as the entries and comments using the desired userpic keyword still exist, when you assign that keyword to a userpic, those entries and comments will use that userpic.
Similarly, when you change your default userpic, all of the comments or entries where you left the userpic as default or selected the default userpic on purpose will now show the new default userpic.