You should be able to find full terms and conditions on the bahn.de web site, but AFAIK these cheap advance-booking fares tend to be non-changeable and non-refundable - i.e., you either travel on the train you booked or you forfeit your ticket.

The regular fare is the same regardless of how and where you book.

In the situation you describe I would get a regular ticket, and buy it at bahn.de to save time on arrival.