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.
All of them have a substantial number of questions, but you are right that spectrum should be consolidated under spectra. Otherwise, I think we should probably leave them be, but spectrometry needs a tag excerpt.