String Replace in Custom Field

[tehQspm]

I'm trying to create a plugin where it'll do the following:

1. Take a person's xbox gamertag (currently assigned to custom profilefield 11)
2. Replace any whitespace in the field with a plus symbol '+'
3. Save the newly replaced string as something I can use in the memberinfo templates

One of the xbox gamercard mods had a link to some instructions that might have been able to help but the link is now dead.

As for the plugin itself, I'm not sure which hook is best to place it in but it'll be either one that activates the code whenever someone updates their profile or member_profile_complete perhaps?

If anyone could point me in the right direction on this I'd be eternally grateful.