Change one thing, then wait a week
Metadata changes only with a version, the index takes days, and only one change at a time tells you anything.
Name, subtitle, keyword field and screenshots change only when a new version goes live, and you can ship a version with no code changes to do it. Apple reindexes within a day or two; positions keep moving for a week, sometimes longer, as the new terms start getting taps or not.
Change one field per release when you can. When the name and the keyword field change together and a term falls, there are two possible causes and no way to tell them apart. Position charts here mark every release so the before and after is visible per term.
Judge at a week, not the next morning, and against three outcomes. Impressions and downloads up: keep going, add the next store. Impressions up and downloads flat: the terms bring the wrong people, go back to relevance. Both down: restore the saved metadata in the next release and start the analysis again. A term that gains and holds is a signal; one that jumps and falls back within days was a reindex.
What no longer works, so you do not spend a release on it: stuffing the description (not indexed), repeating a word across fields (ignored), typos as keywords (the store corrects most misspelled queries now), "free" and "app" and the category name (dropped), and chasing a term whose top 10 is held by apps with a hundred times your ratings.