You can't reuse matrix IDs (yet), as they are the unique key for your message history - it's like email; if you reuse an email address you'll start getting email intended for the original user.

We're looking at fixing this in future via https://github.com/matrix-org/matrix-doc/blob/rav/proposal/r....

Meanwhile, if you want echelon as a username, go register a domain name, run a matrix server on it, and you can have @echelon:echelon.xyz or wherever. Try not to get sued by five eyes though ;P