on Monday I have to fly to HAM, I got a Business Class ticket for the flight at 8:20am but the company who sponsored the ticket wrote me that I can also choose an earlier or a later flight because the ticket is flexible, so I decided to take the 9:55am flight to sleep a little bit longer .
Now my question, do I have to call LH already tomorrow to rebook myself on another flight or is it ok when I arrive at the airport at 9:00am to check-in for the 9:55am flight? The same for the return flight, I am booked on the 7:45pm flight but I think my appointment there is rather short so I can easily catch the 3:40pm flight, do they rebook me without problems on the earlier flight when I arrive ~45 minutes before boarding or do I have to tell them the changes in advance? And do I have the guarantee to get a seat on the flights I want to change to?
Sorry if this is a stupid question but this is the first time I want to change to a flight other than stated on my tickets.