Changing domains in Exchange

Forgive me for my n00b question...

I am supporting an exchange server with 'oldname.co.uk' and the company is going through a name change to 'newname.co.uk'. I created a recipient policy so that all users can receive from the 'newname.co.uk'.

Now I need to change over the outgoing email address over so when people send their email comes from 'newnamme.co.uk' rather than old. Is this just a question of changing the virtual server setting?

I have allready made changes to domain nameserver and MX records to point to our IP.