Display Custom Profile Field in place of Username?

[CYD]

How do I display custom profile field in all the locations where the username is displayed?

For example:
username = john123
customefield5 = John Smith

All posts, forum home page, who's online, PMs, etc...all show "John Smith" instead of john123.

Thanks,

~CYD

(So far I've been able to get this to work, however, threads/forums show the username for all new posts/threads until I update counters -> rebuild threads/forums)