It depends what kind of transition you want.
Create a vertically-subdivided plane-mesh in the XZ of its object space, and place its origin somewhere on its bottom edge-loop. Give it a Simple Deform > Bend modifier, in the Z axis.
- Its X-length will be the circumference of whatever cylinder / tapered cylinder / cone is produced after a 360 degree bend.
- If you put the origin at the center of the bottom loop, the seam will be directly opposie.
- You can freelyG ShiftX move the top loop in Y and Z to produce the desired pitch / height of your cone. The cone will close to a point when the top edge is over the center of the generated circular base. You could put something there to snap to, in Y, if you liked.
As you can see, the cone is produced from a tilted plane.. if you wanted to start with a vertical plane, you could, for example, use a bone to tilt the mesh with respect to its Object space over the length of an animation, while the Bend is growing to 360.