Changing/resetting your phone might be stressful. In this article you will find information which will help you go through this process without losing data in your calendar.
While switching/resetting the phone you should bear 3 points in mind:
1. The pro license
You don't need to pay twice. Please make sure, that you use the same market and the same account as for the initial purchase with your first device. Only then the market will remember you as a real owner of the application and will let you download it again for free to the new or reset devices. The license is normally valid forever without any time or download limit.
2. The data back-up
Do you sync your calendar(s)? If so, just make sure, that the data is correctly synced over to the servers of the sync partner, and it will appear on your new/reset device once you add the account. More detailed information here.
If you, however, have some local calendars, you should remember to back them up as described here.
3. The back-up of app and widget settings