At some point you may wish to redevelop your Pantheon site. You can take advantage of Pantheon's multiple environments by doing your new work in the dev environment. However, you will not be able to apply updates to your live site without also pushing up all of your new code.
You can also create a new dev site and do all of your development independently. You can start from scratch or start by exporting your old site and importing it to create the new one.
Experienced developers might be interested in Pantheon's multidev feature, which allows multiple development branches.
- Pantheon help page: Multidev