A new transition between alternating gyroid (G(A)) structure and cylindrical morphology was observed in thin films for a poly(isoprene-block-styrene-block-(2-vinylpyridine)) (ISP) triblock terpolymer (I/S/P volume fraction = 0.20:0.66:0.14) exhibiting in bulk a Q(214) (space group I4,32) morphology. Thin films were organized using solvent vapor annealing (SVA) and characterized by atomic force microscopy, transmission electron microscopy, and grazing-incidence small-angle X-ray scattering, coupled with several selective revealing methods adapted to the ISP system. After a short SVA time, the bulk morphology was reproduced with the (011) plane parallel to the surface. With a longer annealing time, a P2mm structure with cylindrical polyisoprene (PI) and poly(2-vinylpyridine) (P2VP) domains was obtained. Evidence of domains in transition between gyroid and cylinders was found, suggesting an epitaxial relationship between these two structures. In contrast to the well-known double gyroid (G(D)) to hexagonal (HEX) transition where {121} planes of the G(D) structure are epitaxially related to the {100} planes of the HEX phase and cylinders grow along the < 111 > direction, we showed that the {011} planes of the G(A) phase become the {001} planes of the cylindrical structure (P2mm) and the cylinders grow along the < 010 > direction of the G(A), parallel to the substrate. This was directly related to the three-dimensional topology of the PI and P2VP networks.