Once employed there is a transfer protocol which allows you to request a change of base. However new bases (BCN,AMS) have no tranfer protocol and some other new bases (LIS,OPO,NAP,VCE) have a very restrictive transfer protocol where after a number of years you become eligible to request a transfer.
Good luck to those applying. There is a reason why these new bases are short of internal applicants and the need of external applicants. To give you an example, in my base we have a lot of Portuguese crew, who prefer to commute to LIS or OPO instead of transfering there due to the T&Cs. The same goes for the Spanish.