I've never heard of opening the computer case to terminate a warranty. In fact, even swapping the cards don't terminate it. All these operations are regular maintenance and expected events in the lifetime of a computer. Of course if you botch up and break something, fixing that won't be under warranty.
In fact there are cases of this. Our brand new in house Quantum Computer's warranty is voided if the case is opened or even if you breath too hard on the case. We have to be very very gentle with her.
