I'm able to retag questions on SO. I've just noticed that there exist a push-notification tag with 1700+ questions and then a push-notifications (plural) with some 30 questions.
My thought is that those 30 questions should be retagged to the most common singular tag, and then the plural tag should be canceled.
Is there a way I can do this, with my SO reputation?
I merged the plural into the singular and created a synonym to keep it from coming back. It's not uncommon for the plural of a tag to creep in. The best thing to do is what you've done, which is open a request for it here when you see these creep up from time to time.
Can I batch-rename a tag?
No, for regular Stack Overflow users this is not possible. You'll have to manually edit (or re-tag) each post.
then the plural tag should be canceled
A tag with no posts automatically gets removed in 24 - 36 hours.
A diamond moderator can merge those tags, so you don't need to do that manually. This seems to be a simple case where nothing more than merging is needed.
