Just replace the jar in the plugins folder. It will work as usual

...you never run the last update on a 1.20.6+ server (update was deleted because of the shulker thing - The version String was “1.1.9” - Check with /ver AdvancedEnderchest)

1. It is no problem if you run the 1.1.9 update on a server < 1.20.6!
2. Install this update on your old server < 1.20.6
3. Perform /aec migrate in console
   1. After migration you can still use the plugin in < 1.20.6 if you use this update
   2. A database backup will be created
   3. It is no problem if the server crashes or dies during migration, you can run the command as often as you want
4. Now upgrade the server to 1.20.6+ and use this update. All items will be there
5. Remember: You can not use 2 different versions on 2 different servers that access the same data (this is not new). So you can not run a 1.19 server and a 1.20 server with the same database for example.

...you are running the plugin version 1.1.9 (the deleted upgrade)

1. Install this update on your 1.20.6+ server by replacing the jar
2. Perform /aec fmigrate
   1. A database backup will be created
   2. It is no problem if the server crashes or dies during migration, you can run the command as often as you want
3. Done
4. Important: If you have players with a shulker in your chests, data may be lost (Stacks will be reduced to 1) - But the data will be lost regardless of migration or not. The migration ensures that future shulkers will be saved correctly again. I am so sorry that I overlooked that in the last update of my plugin.