What you have to understand is what a train ticket is: It is a receipt for payment. it is a proof that you have paid do be transported from FRA to Hagen on a particular day. It is not the purchase "a place in a vehicle".
The default is for this "proof of payment" to entitle you to take any train on the route on the day it is valid. In my youth we used to buy tickets for places at the other end of Europe without even knowing any train schedules.
Now Deutsche Bahn also sells discounted tickets where you commit to taking a particular train in return for a discount. The Sparpreis and Supersparpreis tickets need to be used, if possible on the trains you booked.
But from the moment that is no longer possible because of something the Railway itself did (like cancelling a train, changing a schedule, or just running late) your ticket becomes fully flexible again. So you now have a ticket FRA to Hagen that you can use on any train on any allowed route on the day it is valid. So you can really now just jump on the first train going your way once you are at the station.
The only reason to go to the ticket office is if you have a reservation for the now cancelled train, and you want to have it transferred free of charge to another train.
Now you state that you want to take the "left bank" of the Rhine. Which trains did you originally book? Most trains from FRA to Hagen take the new fast line.