All depends on the country where the airline/aircraft are registered. I assume you are talking about the "big" airlines-- EK, EY, GF . . . etc.
In any case, there is simulator and type training involved, which is a requirement in any case when you change employers. The license is then awarded upon successful completion of your simulator check. Worst case scenario, it also will include an exam on the air regulations of the country-- some do not, however.
It is not much more than if you were to change from one company to another in your own country.