This question is all about finding out the best way to handle merging similar terms.
Let's say you have 10 posts tagged as
Barack Obama and 20 posts tagged as
and you want to group them all under
Barack Obama and get 30 posts for it and get rid of the tag
Surely, there will be some changes to the wp_terms, wp_term_taxonomy, wp_term_relationships. But in doing so, which APIs should be used and under which order so that while one tag is being gotten rid of, all the posts associated with it will be tagged as the other one.