Replace all usernames by "True Name" Field for 3.6.x

[gbml4u]

Just a small questions

I've created a field : True Name (required at registration)
Now, i want to replace all usernames by this field.
http://hn-ams.org <== they can do this.
(I've just done it only at postbit_legacy & postbit templates)
Can someone help me ?
Thanks.

Sorry about my English !