Polymersomes, polymeric vesicles constructed of block copolymers, can undergo a sphere-to-tubule transition under the influence of a chemical modification of the polymeric bilayer. A strain-promoted alkyne-azide cycloaddition (SPAAC) reaction between azide handles inside the hydrophobic domain of the membrane and an excess of a bicyclo[6.1.0]nonyne (BCN)-cross-linker causes the vesicle to stretch in one dimension. Tubular polymersomes up to 2 μm in length can be obtained with this shape transformation. The introduction of a cleavable cross-linker makes this process reversible and opens the way for future drug delivery applications.