Simply start with the trial license today. The trial license lets you use the full capabilities of CA Agile Requirements Designer for 30 days.
You can always re-open the License Wizard from the Help menu to switch to another license. After 30 days, you can either switch to the viewer license to use CA Agile Requirements Designer indefinitely in read-only mode; or you can decide to buy CA Agile Requirements Designer and request a named license, or set up a concurrent license on a server for several users.
To extend the trial period or to purchase the full product, reach out to the Global Service Center.