YMMV but I always make my onward reservations immediately upon arriving at each location. I am already at the station so don't have to make a special trip. Waiting until the last minute when you need 3 together might become problematic.

Don't stress too much about missing a departure due to changing plans. There is no penalty. Just abandon the reservations and take a later train. You can make new reservations or try to walk-on the unreserved cars. Be early if you do that as the seats often go fast, first-come first-served.