I know that you can triangulate a selected portion of a mesh in Edit Mode by hitting CtrlT. This converts all quads and n-gons into tris. How how can this be reversed, combining coplanar triangles into quads and n-gons?
You can untriangulate the selected portion of a mesh by hitting AltJ. This will only create quads so some manual cleanup may be necessary if you need n-gons. You can do this by selecting the leftover edges and hitting X and clicking Dissolve Edges.
You can use the decimate modifier using the
planar option and apply it.
You can also do it through Blender's Python API if you want to get quads: