Yes, because even if you upgraded your Pocket app to unlock more features and maps, it remains the Pocket app. Unlike a PC or Mac, there is no way on iOS to replace an app by another. So you need to purchase the Ticket to Ride app (which is why we reduced its price temporarily) and use your Days of Wonder account as a conduit to move your maps from the Pocket app to the Ticket to Ride app.
So after you are done with the migration process in Pocket, make sure you log in with your Days of Wonder account in the Ticket to Ride app to recover all your maps (and also get the Pennsylvania map for free as a compensation of your purchase).
This is accomplished by going to the Settings screen inside the Station, and pushing the "Login" button.