As it's obviously the same train, you do not have to do anything. If you're worried, you can check on bahn.de the seating map

Btw optional seat reservations in Germany, Austria and many other countries are way more expensive through eurail.com

- 6-8€ + 2€ booking fee

- 3€ per train on tickets.oebb.at (free with a 1st class pass)

- 4.50€ per journey on bahn.de (5.90€ 1st class)

If you need to make other reservations, have a look through this link to help you. It's usually best to avoid eurail.com to book seats because of the fees and sometimes higher prices. https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

Thibcabe, thank you for the reply. I was in the process of contacting Deutsche Bahn, but saw a reply from Eurail (congrats to Eurail for a speedy reply!). Even though my original train didn’t show up when I searched (to add the trip to My Trips), If you purposely choose that time, the train did come up. 

Interesting that it didn’t show automatically. But the good news is that the original train time is running, so my seat reservations don’t need to be changed, or re-purchased. Phew!

Are you sure? Where did you check? Eurail.com is not reliable. It's best to check timetables on the operators’ own websites, e.g. on bahn.com in this case.