Well, you have Impulse right? Just install Impulse on the new computer and download it from there. You don't have to purchase the game again.

In that case, you just might find that there are a whole slew of updates to Sins you've been missing out on. Depending on when you first installed it.

Usually people that mod want access to older versions as time goes on. Impulse always automatically assumes to give you the most current version however. What you can do is archive your installation so that you may roll back to a previous version if ever needed.

To do this, click the Impulse orb in the upper left and then click 'Preferences'. Click the 'Archive & Install' listing on the left. You can toggle Impulse to automatically archive your install before downloading new updates. If you wish to do it manually, you can right click on the game from the 'My Games' tab and choose 'Archive Application'.