if some old fucntions are not supported in version then how to chnage those functions?

HI Champs,I want to upgrade sql server 2005 to sql server 2014 . In case if some old function may not support by new versions, then how can i subsitute alternate for that ?IS there any possibility like some old functions are removed in new version ? if yes then how to upgrade such lines of code ?Regards,Ravindranath.S