There are a lot of those. A lot of vets don't do specific testing, but it CAN be done to narrow down which respiratory disease your birds have. You might want to see if you can find a lab in your country that will do testing for you. Here in the US, we have a lab that accepts swabs from the throats of the sick birds and will run a panel to determine which disease it is. You may have something similar in your country.
Depending on which disease it ends up being, yes, unfortunately it is permanent. Denagard has been said to be able to be used against Mycoplasma gallisepticum with an aggressive treatment and dosing plan to maybe actually cure the birds, though I cannot say for sure it actually works and the birds actually stop being carriers. With other diseases such as coryza, Ornithobacterium rhinotracheale, ILT, etc, there isn't a way to cure it; you can only treat the symptoms when they come up.
Infectious bronchitis, however, DOES pass from the chicken after awhile, and they stop being carriers, usually within a year of the infection. This one is a virus, though, so if your vet gave you antibiotics and they worked, then this is pretty much definitely not the disease they have.
So if you can find out which chronic respiratory disease they have, that can help you a lot as far as treatment.