Yes, Ranking changes overtime. Let me give you 2 real-world examples.
My username is the abbreviation for my channel. I use the first three words as a tag and try to rank that tag in search. I have a few other competitors that do the same. Last week I had a livestream that did really well and I was ranked (I think) 3rd for that tag. By yesterday when I did my livestream I was no longer ranking that tag. Depending on the performance of yesterday's livestream, I may end up ranking that tag again. I won't change that tag because I want to have it in every video to maximize my chance of ranking that tag.
Example 2, I have a friend that I helped put out a parody video on an extremely popular topic. His managed to rank all but 3 tags in the top 5 of search. But it was a popular topic. And after two weeks there were a dozen other parody videos on the same topic. He still ranks top 5 for about 1/3 of his tags but the rest either rank low or don't rank at all.
Thing change. But give your tags some "air" and don't rush to change them constantly. Pay more attention to overall performance, not just tag rank. If you video isn't getting views, check the title, thumbnail, description, and tags. Changing them may cause an increase, a decrease or have no impact, so make changes once a video's performance flattens out.
Hopefully this answers your question, so I am going to mark this answered and close to further comments.
