Yes, this should work.
For future data moves: There is another method, which involves directly editing the Plex database. Thereby changing all file paths to the new storage location: [HowTo] Plex database modification - Moving media the right/wrong way - #10 by jelwell
Of course, in an ideal world the above workarounds wouldn’t have been necessary. But I assume that your old movie library has been created with the old Plex Movie agent.
Then the library got converted to the new agent, but the necessary Metadata Refresh to finish this process was never performed.
So when you added the new path, all the files in there got matched with the new agent, while the old items were still matched to the old agent. When this happens, Plex cannot recognize them as the same movie and creates new items, which of course don’t share their ‘Added at’ dates with the old items.
