Is it not recommended to change the object of meditation during a short session?

Sometimes I tend to switch my attention from the nostrils to the "up and down" of the chest/stomach It depends on how well I'm breathing (I have serious problem with the nostrils, it can only be 100% fixed in a surgery) I do prefer the nostrils, however this change is commom everytime it becomes impossible to keep breathing throught them (so I swith to the mounth and focus on the chest) Is this switch not recommended for any reason, for instance, is this a problem to reach right concentration?