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Page Authority is just a marketing metric made by Moz and won't update until they re-run their index, which I believe is still on a ~monthly timeline. Just because Moz says a page temporarily has a PA of 1 doesn't mean Google sees it that way. If you have a good reason to change a URL structure, feel free to do it as long as you have 301s. Do not change your URL structure for small, un-needed reasons, such as cramming a slightly better KW variation in the URL.