Do you have a backup of that database so that you can use it for testing what your are trying to achieve?
If you receive results from that query, you should be able to make out a pattern of your username convention in order to rule out to choose for a replacement username that scrambles up your database/tables. Might be worth looking at variables which you can add like sam smith %real1% ... to sam smith %real100% to eventually in a second step replace 1-100 with the ones you found are the correct ones to be replaced.
Once you have reviewed your data you can then try to test different queries, joins, etc. to get to the custom script you need.