Can I change my username on apple discussion?

Sadly the answer is NO.

You are unable to change your username on Apple Support Communities, once you have created an account. The only way around this would be to create another Apple ID, and then you would be able to create a new Apple Support Communities account and pick a username of your choice

You can have many AppleIDs, but only one per email address. Create the new ID under new email and log in here. You do not need to change the AppleID in your devices (iPhone, Mac, etc).

Start at appleid.apple.com and select "create".