Minecraft player UUID changed after online-mode migration

Minecraft server player files are named after UUIDs, not usernames. If the authentication mode or account identity changes, the same visible name can point to a new file while the old file still contains the inventory and position you want to recover.

Why the same username can have two UUID files

Online-mode servers normally resolve players through Mojang or Microsoft account identity. Offline-mode or cracked setups derive UUIDs differently. Switching modes, changing proxies, or changing the account used to join can therefore create a second UUID-named file.

Check online-mode before touching files

server.properties
online-mode=true|false

Find the old and new playerdata files

1. Stop the server and copy the entire world/playerdata folder.
2. Check usercache.json and server logs for the username-to-UUID mapping used by each session.
3. Open both candidate UUID files and compare LastKnownName, Pos, Inventory, and other recognizable data.
4. Treat the file containing the expected inventory as the source; never choose by filename alone.

<world>/playerdata/<old-uuid>.dat
<world>/playerdata/<new-uuid>.dat

Back up playerdata, advancements, and stats

Rename or copy the old data safely

1. Keep the old and new files in a dated backup folder before making any replacement.
2. Prefer copying the needed data into a test file first; preserve the new file until you have verified the result.
3. If you must replace the new file, save the previous version as a .bak file rather than deleting it.
4. Do the equivalent comparison for advancements and stats only when you understand that those folders use the same UUID identity.

Verify the player after restart

Start the server only after the replacement is complete, then have the account join and verify the exact fields you intended to restore. If the server creates another new UUID file, stop again and resolve the authentication or proxy configuration before trying another copy.