I am using QGIS 2.16 to try to convert polygons with 3d vertices to polylines with 3d vertices. Using the tool Vector -> Geometry Tools -> Polygons to Lines writes out the polyines, but the z information has been removed. Am I missing something or is there another workaround in QGIS to achieve 3d polylines from 3d polygons?
If you can upgrade to 2.18 you could use the "boundary" algorithm in the toolbox (https://www.qgis.org/en/site/forusers/visualchangelog218/index.html#feature-geometry-boundary-algorithm). This algorithm fully supports z/m values and curves, and given that the boundary of a polygon is the exterior and interior rings you'll get the same result as converting polygons to lines.