You don’t need to repurchase StartupPro, you pay once and you’re in for life. Every future upgrade is available to you without further purchase.
Just resent the receipt/download link. Bookmark this link in your favorite browser and you can download the latest copy from there whenever you need.
Also, in the newer versions we’ve now added an automatic update feature so that you can update your theme right from within your WP admin.
As far as moving your entire WordPress install to a new domain, please see: