As it stands currently the simple answer to this is that we are unable to do this unfortunately.
The reasons for this are that particular data gets pulled from researchers profiles in the background and is used for associating the researcher with their submitted reports, payouts being allocated and awarded, assigning reputation points, and leaderboards.
Researchers are welcome to open a different profile should they want to use a different username. If the email address from the current profile would like to be reused the only option we can suggest is to delete the current profile to unallocate the email address, after which this can then be re-registered with a new profile.
Reach out to the support team and explain what you would like to do.
Please be aware this would mean starting again with a new profile, we are unable to transfer any submissions already made, nor reputation points or bounties owed from the original profile.