Test domain for Wordpress. When I'm ready ... how is it moved over

I'm about to make a testing area (on a subdomain test.mysite.com) for a Wordpress site, my only concern is that when I'm done and it all works, how do i move the whole thing to the main site? (i.e. www.mysite.com). Will it still work? that is. will the thing be set up to only work within say test.mysite.com

One last thing - its a BT hosted site (over in the UK) and I'm hoping it will go to plan. Anyone here have any experience of using BT hosting? I'm feeling nervous about how this is going to go - its like dealing with a mammoth or whale or something huge that does what ever it likes. Any ideas appreciated

rgds