Hi Frank,
fortunately there is a trick to keep all the constraints… 
It is a hidden feature that I’m happy to disclosure 
The ECAD/MCAD collaboration works in that way:
- Start your board in KiCad designing a very basic pcb edge (a simple square or even a circle) and add a Grid Origin to the board
- Pull (load) the board (even without an edge or with a ‘proto’ square edge) in FC
- Design your pcb Edge in FC using all FC Sketcher facilities (included Ellipses or BSplines)
- Save your design with the Sketch as FC file
- Push your Edge to a KiCad pcb file
- Design your tracks and place your modules and 3D parts in KiCad
- In case of some Edge modification, Pull the board in FC (that will load the pcb Edge loosing your previous assigned constraints) loading the new position of parts
Here comes the tricky part: - Load the previous release of your KiCad board as FC file
- Copy the Sketch of your previous release to the new one (Copy/Paste the Sketch between the two FC files)
- Delete the unconstrained Sketch
- Make the Edge changes to the previous release Sketch
- Push back the Sketch to your KiCad board.
- Save the new release of your pcb Edge as FC file.
This is a Push-Pull procedure with a sort of versioning of your Edge and PCB tracks.
One important note:
You need to assign into your KiCad pcb file a static design reference since the beginning.
The reference is:
a) Grid Origin (suggested)
http://docs.kicad.org/stable/en/pcbnew.html#_setting_coordinates_origin
or
b) Drill and Place offset (this will affect also Drill output of your gerbers)
and configure also StepUp to use Grid Origin (as suggested in the Cheatsheet)…
in ‘ksu-config.ini’ set:
[Placement]
placement = usegridorigin
;; placement options
;placement options: useauxorigin, usebaseorigin, usebasepoint;x;y, usedefault, +autoadjust
Following this versioning procedure you can keep your Sketcher always up to date with your board, and keep also the original geometries and constraints that will be lost in KiCad.
M