It shouldn't really make any difference whether you uninstall Win7 Home from your old disk... in the first place, but like others have noted, it would be wise to disconnect it. Later on, when you get your new one started, I would think it would be good to re-organize your disks, and get rid of the old installation. Meaning:
1. Re-partition the disk with your old OS / together with
2. Moving files and folders, if necessary hence and forth --- you may need to move them to the new disk --- partition the old disk --- move them back to the old disk --- it may be a task. BE CAREFUL THERE, I just lost some 250Gb doing it... not funny!