Perhaps more logical/philosophical rather than math question. In binary classification setting, classes are most often not symmetrical and one of them is considered to be "positive" or "success", while other one is "negative" or "failure". This fact is used for classification with skewed posterior probability threshold, it defines such notions as true positives, false negatives, etc and corresponding evaluation measures.
Now the question - does the notion of success class belong to the model, i.e. once you have fitted the model based on the training features/response - success class must be defined at this point and changing it afterwards is a violation? Or one could have a fitted model and still change the notion of the success class - say for predicting new data? At this point the training data/labels are N/A, and classifier has done its job - so does it mean that the success class notion should not/ could not be changed after this point?