For some reason, we are experiencing a bug where inventories get wiped when players change their username. We are not quite sure where the problem occurs or how we can solve it just yet. This is why we advice you to not change your username right now.
If you do want to change your username right now, make sure to log in BEFORE changing your username, and putting all your items from both your normal inventory and enderchest in a chest.
This bug only seems to affect inventories, so the following items should not change when changing your name;
We're sorry for this inconvenience, and we are looking into this problem and a solution. Again, if you want to change your name make sure to place all of your inventory (including enderchest!) into a physical chest in order to save it.