The functionality of a downhole tubular expansion tool is switched by: -providing the unexpanded tubular with an oversized section (7) having a larger internal width than other (non-oversized) sections of the unexpanded tubular (6);-providing the expansion tool with a control sub (1) comprising a trigger assembly (2) that is pushed outward within the oversized section and thereby switches the functionality of the tool, for example by contracting or expanding a collapsible expansion cone and/or opening a valve.