Switching letters is called spoonerism

SPOONERISM. [From the name of the Reverend W. A. Spooner (1844–1930), Dean and Warden of New College, Oxford]. The transposition of the initial sounds of words, as in ket of seas (set of keys). The eponymous Spooner was reputed to make errors of this type, and a number of utterances are quoted as ‘original spoonerisms’: a well-boiled icicle (a well-oiled bicycle), a scoop of Boy Trouts (a troop of Boy Scouts), and You have hissed all my mystery lectures and tasted a whole worm. http://www.encyclopedia.com