People ask all the time about moving media content. I think the reason they ask is because I think this article is misleading:
https://support.plex.tv/hc/en-us/articles/201154537-Move-Media-Content-to-a-New-Location
You can't move content to a new location and expect any of your manual changes to apply to the moved content. If your files are all named perfectly, then sure, Plex will simply rematch your files. But any content you added posters to, edited titles or tags, or did a "fix incorrect match" on won't be matched correctly no matter how closely you follow the above article. Please correct me if I'm wrong, but I have tried following the article multiple times and everytime I've reverted to my backup because my manual changes haven't been noticed.
What I want to do is (backup, then) open up the sqlite database and modify the location in the database itself. I'll report back how well this fails. :)
Joseph Elwell.