By using the concept of perceptual region – an essential part of the identity of a region and a part of every person’s mental map – this paper demonstrates a way to examine the understudied transformation of (the identity of) a region and, specifically, its territorial shape (boundaries). This concept effectively fuses the “institutionalisation of regions” theory and the methodologies of behavioural geography. This case study of the perceptual regions of Bohemia and Moravia shows how and why these historical regions and their boundary/boundaries developed, after a significant deinstitutionalisation by splitting into smaller regions in an administrative reform. Many people now perceive the Bohemian-Moravian boundary according to the newly-emerged regional boundaries, which often ignore old (historical) boundaries. Thus, the territorial shape of Bohemia and Moravia is transformed, with the Vysočina Region emerging as one of the new regions to witness the most eroded consciousness concerning these historical regions and their boundaries. The impact of administrative reforms on the perception and thus also the transformation of regions and boundaries is obvious, but the results also suggest that the more radical the administrative changes (in terms of toponyms and boundary mismatches), the fuzzier the collective perceptions of historical boundaries become, as well as peoples’ consciousness of historical regions.