It's good practice to have a separate Data drive (e.g. D:), install the Symantec Management Agent to D:\Program files.
Once you've bilt your new Site Server and installed the Symantec Management Agent make sure you have all the Site Server pre reqs installed, .Net, IIS etc.
Use Site Management to add the new server a a Site Server, you might need to wait for Basic Inventory to be processed and the relevant Filters to update so the NS knows it's eligible to be a Task and Package Server.
Once it's installed Task and Package Services and reported back you'll need to make sure all your Packages are set to use it.
Then I'd wait a few days for the Packages to replicate and to test it works.
Once you are happy you can use Site Management to remove the old Site Server. You could move it to a dummy empty site for a few days first so most Task Agents still using it have time to move across.