After a new user registers for a forum they receive a welcome email to activate their account. The signature of this email appears to be my original site name. Since then I've changed my site name slightly. Is their any way to change the signature of this email or update it so it is in sync with my current site name?

Try this:
ACP > General > Messages and Emails > Emails and Contact
Then just change the text you currently have in the e-mail signature field with whatever you want. c:

Doh! Thank you