Incomplete worlds or ambiguous actions are topics that encourage research in theories about reasoning and action (Brewka and Hertzberg 1993). Driankov's model-based approaches propose to deal with the problem using Belnap's four-valued logic. Incompleteness or ambiguity is clarified by postconditions that apply whenever persistence conditions are inhibited. In this paper, Driankov's semantics is used in a formalisation that captures the dynamic character of knowledge and belief. Ambiguous knowledge is present in several kinds of situations that formally correspond with knowing that a disjunction is true, but it is not known which element of the disjunction makes it true. We define an a posteriori knowledge operator that allows to extend knowledge from ambiguous knowledge or undefined information, being in the meanwhile, potential knowledge and/or belief. The expansion operation of AGM paradigm (Gardefors 1988) is used to explore the possibilities from ambiguous information.