The company has drawn up a shortlist of Campbell Doyle Dye, Vallance Carruthers Coleman Priest and Michaelides & Bednash. The review is being run through the AAR.
The sign has traditionally featured animated variations of Coke's logo, but the company is looking to make more use of the medium and will use it to showcase specially tailored interactive content.
The company is considering plans to double the sign's size. This would involve buying out the other advertisers who use the site, including Samsung, McDonald's, Fuji and Sanyo. The strategy is to make the sign a newsworthy landmark.
The site is owned by Land Securities. Space is sold by The Marketing Department.
Three years ago, Coca-Cola upgraded its display from a static neon sign to digital. Last year, it revamped the site again, replacing its projector installation with an LED daylight display system.