Hi jucker, also those steps that you show of initially drawing a polyline and then switching it to be a smooth curve would be an unusual way to approach it in MoI - in MoI if you know that you want to have a smooth curve just draw it that way with a control point curve right from the start...

If you have a straight section that you want to switch to being bulgy, then you can use Edit > Add pt, to add some control points to that segment, it will then be bendy in the same way that you show there:

Starting with this polyline:



You can add 2 control points to this section here:



Then when you move those you will get the same kind of thing that you were showing:



If you want to make it a smooth piece connecting to the next section, add some more points to the next section too, then delete the corner point at the juncture between the segments:



That will fuse together those 2 segments into one long smooth segment and you will then have smooth curve editing behavior on that longer sequence of 6 control points:




But it's not really so typical to convert things back and forth in this way - usually if you know you want a curvy piece you should just draw it directly by using Draw curve > Control points, then there is no conversion needed. Also it's pretty unusual to need to go the reverse way and try to form something made of straight lines from an existing curve, it just does not come up all that often and when it does the simple way to do it is to just draw in some new lines how you want them and delete the curvy piece.

- Michael