What I have gleaned from reading many threads on this topic is that reservations are actually canceled and rebooked when we chamge them. It seems to us guests that we’re just adding or subtracting days to a reservation, but that may not be how how their system actually works.
I have a similar situation to yours at the BWI, but I wanted the longer stay, just wasn’t sure I could keep it. In the end, I didn’t book the entire 10 days, only the week I knew I could be there, and the three iffy days at the Hilton, where I’d be able to adjust it as needed. I just wasn’t sure I’d be able to rebook if the vacation changed fro 10 days to 7, and didn’t want to risk that week at the BWI. But there are only 10 of my room type- if there are more of your room type that might factor into your decision. You can check the TP room finder to figure that out.
If the reservation system is already telling you that a shorter stay is not available, I wouldn’t count on it being there later. That seems counterintuitive to me. Things generally don’t get easier to book as time goes by, they get more difficult. Maybe if you called and asked, you might find out a little more info that might help you decide.