Through a discourse-grounded internal reconstruction that aims at capturing the emergence of grammatical structure, the study examines the development of the subjective epistemic particle jestli ‘[in-my-opinion-] maybe’ in conversational Czech. Through internal reconstruction, the change (syntactic complementizer > speaker-centered epistemic contextualizer > subjective epistemic particle) is presented as a metonymy-based conventionalization of a pragmatic meaning implied by certain tokens of indirect Y/N questions into a new modal meaning. Taking a Construction Grammar approach, so far largely untested on diachronic data, the point of the analysis is to show that we can engage in a systematic treatment of the gradualness of change, by (i) combining the ‘holistic’ (constructional) dimension with the internal, feature-based and discourse-motivated mechanisms of complex grammatical shifts, and (ii) appealing to the explanatory potential of general cognitive and communicative principles as they manifest themselves in natural discourse. I also propose a formalism for representing the transitional nature of intermediate patterns.