Recently, I created the tag hr-diagram as there are many questions regarding it. I went ahead and edited a few questions to include this new tag. Would it be a good idea to retag all the 82 (and counting) results that include the phrase HR diagram, or should we keep them as is and let new questions trickle in with this tag?
Manually retagging more than 4-5 questions in a big batch is discouraged because unfortunately it clutters the active questions feed. But if you want to slowly work them in over time, I don't see anything wrong with that. Since you use an abbreviation you should probably create a tag excerpt that describes the tag and spells out Hertzsprung-Russell diagram.