The half-elf gets a racial power that lets him take an at-will from a class not his own and make it an encounter power. Does that mean it replaces his class encounter power or that he gets an additional encounter power?
You seem to be talking about the Dilettante. this does not replace anything. It is a bonus Racial power. The first Player's Handbook has the details on page 42 as does Player Essentials: Heroes of the Forgotten Kingdoms on page 252.
If it replaced something the text would say it.