The Instax is a camera which uses light-sensitive film to capture photos. The film packs include a cover that prevents light from reaching the film inside while it is being transferred from the package to the camera.

Once the pack has been installed and the camera has ejected the cover, the individual sheets of film are exposed, allowing light to reach the one at the front when the shutter is opened. Removing the pack from the camera exposes the sheet at the front of the pack to light and ruins it, so even if you were already able to convince the camera that it didn't need to eject the cover, the sheet of film at the front of the just-installed pack would be useless.

The only way you would be able to accomplish a swap without destroying one exposure would be to do so in a film changing bag. Packs not on the camera that don't have the factory-installed cover would have to be stored in light-tight containers. By examining the pack, you may find that it's possible to re-install the cover after it has been ejected, but even if you are able to do that, I would still recommend storing partially-used packs in light-tight containers.

After swapping the pack, you would need to take one picture to determine whether or not the camera opens the shutter when the exposure counter is in the S position. (Given the simplicity of the camera, this is a possibility.)