If you have a project building on CircleCI 1.0 and want to try CircleCI 2.0 you can continue to build the project on 1.0.
Create a new branch, and replace your
circle.yml config file with a
.circleci/config.yml file with 2.0 config. All builds on other branches will run with 1.0; the test branch will build on 2.0 while you try out the new config. The docs have migration information: https://circleci.com/docs/2.0/faq/