can i change the key battery for a 1998 ford escort?

i recently bought a ford escort (1998 J Reg).the battery in the key doesnt work so i went somewhere to get it changed today.they said they cant change it until i find out if the battery can b changed because some cars use a code after the battery has been changed and if this is the case and i dont know the code then i may not b able 2 open the car afterwards.does anybody know if i there is a code and if yes then what is it please?thanx.