As an FYI, there is a bulk tag editing mode in the UI, but it was a bit tough for me to find. Go to YOURSITE.com/tags/none and then click the 
button and then you should see checkboxes. Now you can select which ones you want to change/append to, and then click the 
button and you should see a menu with some options.
These shouldn’t change the updated-timestamps on the posts or anything.