The Pete Best: Katie Lee was the hostess of season one of Top Chef before being replaced by Padma Lakshmi. Katie was brought back as a guest judge in All-Stars.
Kelly Choi hosted the first two seasons of Masters before being replaced by Curtis Stone, who is incidentally the first male host of any of the shows.
For Top Chef Canada, Season 1 hostess Thea Andrews was replaced for season 2 by Canadian actress and former model, Lisa Ray. Though in this case, it was because Thea was pregnant, as shown when she guest hosted for the third episode of season two.