Not a bug but....

Apr 30, 2019
Okay so this isn't a bug or anything, but I don't really know what this would be considered so I am writing it here.
I just changed my username from BoxOfCheezits to Treadmill47 and the server recognized it as me using a different account so is there any way I can get my stuff back from my old username, or is that gone? I only played prison (I was a criminal I think) and skyblock (I shared an island with ochoaboyy and vaguelylargenuts).
Obviously if you need me to provide proof that its the same account just let me know.
I appreciate any help that can be provided for returning all my stuff from BoxOfCheezits to treadmill47.
Thank you!