existing website

I have an existing very dated website that was inherited and that we no longer want to use so I have started afresh with PMP.

my question is I am about to shut down the old one.  I have the domain access set up on fasthosts with the PMP info in place.  Do I have to go live under the PMP web address then by linking the domain it will change it to the current existing domain ie removing the PMP part of the web address PMP automatically gives you?  I don't want to go live until I know this is what I need to do and this is not clear from instructions on the site?  many thanks