If i was to purchase a new computer how do i go about registration of my software on my new computer, do i have t unistall my current copy of sonar x3 then download it on my new computer then use my registration codes?

If you intend to give the old computer away, or sell it, yes you would need to remove Sonar to be able to install and use on the new computer.

You would use the same reg code as you did when you installed it.

But if you keep the old PC and use it yourself, then you could keep on using the installed version of Sonar in it along with Sonar being installed on the new PC. As long as you don't use both at the same time.

Sonar has a very user freindly registration process that is based more or less on the honour system. They are only asking that you be the only one who uses it. Once you have your 2 numbers you are free to re install it over and over for eternity. 
 
But Melodyne will screw you around and you have to put your computer on the internet to register it and they only will allow this 2 times. 
You will have to cancel a registration number before you retire the computer and they will let you use that one again. I had a hard drive die and contacted them and they sorted that out in a friendly manner. But I still hate the on line part as it requires moving my DAW to where I have cat5 internet in the house.