Questions can be and are retagged by Experts, Editors and Moderators, and PM himself of course, if we feel the tags on a post are inappropriate, wrong or do not correspond with the question asked or the post is missing relevant tags.
It should be noted that tags are irrelevant on RMT.
So to be able to retag any question you're going to have to at least get Expert, which is granted by a Moderator when someone achieves 6000 Points on PokeBase.
On a final note the question you were referring to ("NU Counters to Miltank") has been retagged by yours truly to avoid further confusion.
Edit: Sempi retagged the one with 'lol'