In order to change the Scheme associated to a project:
1) Go to Project level settings from the top settings icon.
2) Once in the Project level settings screen, press the Settings button for the desired project.
3) Click on the Manage Status Set Scheme tab.
4) Click on the Switch Status Set button.
5) Select the Scheme you want to switch to, and press Associate.
6) Since a Scheme is already applied to your project, and you want to switch Schemes, you are required to map statuses from the old Scheme onto new ones. What this will achieve is, it will preserve your previous work as all items under old statuses will now appear under their new counterparts. To do this, you must select a new status against each status by selecting it from the dropdown menu next to the status name. This must be done for all statuses for all items. E.g. If you replace the Status New with In development, all items that were under New will now appear under In Development status.
7) Press Done and you are finished.