This paper isolates four parameters that guide the historical change of word classes: the quantificational parameter, the directional parameter, the preservative parameter and the temporal parameter. These parameters are involved in the organization of seven case studies in East Asian
languages. Based on these case studies I define four diachronic tendencies that apply to East Asian languages and perhaps beyond: (1) the greater the size of the target word class, the lower the number of new acquired meanings; (2) if a word class engages on a path of change, then the greater
its size, the more likely it is that the process of change in which it engages will be lexicalization; (3) in a typical process of grammaticalization relatively more meanings are generated than in a typical process of lexicalization; (4) processes of grammaticalization represent temporally
short processes more often than processes of lexicalization.