The rolling stock they planned to use on that day was not available. There are a variety of reasons for that, e.g. it was used on a delayed route the day before and has not arrived at the destination or there was unplanned maintenance. If that happens, the train is usually replaced by one of the same kind, which is entirely transparent to the passenger, i.e. you would not notice. In your case, they did not have rolling stock of similar kind available. So why did they assign a new number to that service?
Foremost, the train number is used by the dispatcher and the driver to reference the internal schedule of a train. For safety and operational reasons, once a number has been assigned for that day it can't be transferred to rolling stock with different performance parameters (e.g. speed, brake type, clearance gauge). Only additionally that number is also used in communication to the passenger.
In cases like that Deutsche Bahn used to have diverging train numbers in internal and external communication. As this led to confusion by staff in stress situations, this is now avoided and the passenger is communicated the same train number that is used internally.
Seat reservations do not play a consideration here, this is done for purely operational reasons. Announcing that all seat reservations are cancelled is easily be done in the system without assigning a new number. The latter involves a lot more operational units (namely both the train operator and the network operator which are separate companies), has costly consequences, and would not be done purely for reservations. This is for example seen when an ICE 1 train is replaced by an ICE 2 train. Those are operationally interchangeable (internally called ICE-A) and thus no new train number is assigned. Nonetheless, as the seating plan is different seat reservations are cancelled which is communicated to the passenger.
This is all in sharp contrast to the way an airline would handle all of that (switching between Airbus and Boeing aircraft or even different callsigns do not result in changing flight numbers). In the world of German rail, internal operations and passenger communications are much closer aligned.
