How to Replace a Mesh
There is an option in Cascadeur to import any custom mesh to the scene if the skeleton bones match. This way you can replace current mesh.
1. Open a character with the same skeleton in Cascadeur.
2. Switch to Mesh Mode.
3. Select the mesh and hide it by clicking on Invisible in the Basic tab of the Object properties panel:
4. Switch to Joint Mode.
5. Select the Joints.
6. Select Import → Add model to selected from the File menu.
7. Choose an FBX file with a new mesh
There should be a new mesh on the scene now.
If you import several meshes like this, you can easily switch between them by hiding the unneeded ones with the Invisible button