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?
The simple answer is it is not. Don't change it.


If you concentrate on breathing for a while you mind will play tricks
like, Now the it is time to change the session but it is not. Buddha
has taught not to fall for that kind of feelings.
Even if you try to master elements it the same. don't fall for that
feelings. Possibly if you do it for some time it is likely that you
will get fame, but it is temporary that is a test for you. If you
simply stop everything fades away sometimes you will fall than you
were before.

Read VipasanaBavana from Balangoda Anananda Mithree thero for more info