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Since purchasing my BATC membership my call sign has changed from 2e0ssm to M5SJM. Is it possible to change my username to my new call sign so I can log in with that instead of my old one?
Congratulations on your Intermediate Licence.
Unfortunately, Username and Membership number are fixed and cannot be changed. Your best option is to wait until renewal time, and then rejoin (not renew) with your new callsign. You will also get a new membership number if you do this.