Thanks everyone! Just for future reference if other people should run into the same problem and find this thread: I did not do any changes to the reservation, I did not request any Upgrades, I was very busy with reaching my MarriWood Lifetime Platinum so I only opened the Hilton App once after making that reservation to book another 2 nights in another city, and then never touched any of those reservations (or the app) again until I opened it today and this thing had happened.
I
just called the property as two of you suggested, they confirmed the rate I had originally booked and said "must be an Hilton Honors error when they transferred it during the System update". After more than 1.000 nights in Hotels I have seen a lot so I just wanted to be sure and also asked them to please
email a confirmation to me. It took 2 minutes for the email to arrive, which now had the upgraded room and the old rate (like it should be and always was with all my other Hilton reservations). Very friendly call on both sides, took 3 minutes to get someone on the line and then less than 60 seconds to resolve it (+2 minutes for the email to arrive).The app is also showing the upgraded room and the correct rate (the original price I booked) again now.
Now let's hope they'll upgrade me further on Arrival
Again for future reference: Did something like this happen to anyone else, like to dinotoon and me? What did you do to resolve it?