I was going to say just install the new template as you would any other extension and change it to default in the template manager, but they run on different versions of gantry. I believe Gantry is backwards compatible, but can't state that as a fact.
So I would set up a test site, install RT Cerulean via the Rocket Launcher as you did before, then add the Kirigami Stand Alone -
www.rockettheme.com/joomla-downloads/club/3053-kirigami
- just the template w/o Gantry and see what happens.
If you don't want to bother with a test site, at least use Akeeba to back up your live site before you try so you can restore the site if necessary.
The one thing that concerns me is that these templates run on different versions of Gantry. Why is that? Are there plans to upgrade older templates to newer versions of Gantry? Would I be loosing anything by using an old version?