Odyssey use same saves. From what I've read till now I would assume that the saves will be overwritten by EDO hance unusable by Horizons.
Source: https://www.reddit.com/r/ProgrammerHumor/comments/a2c4gg/quality_assurance/?utm_medium=android_app&utm_source=share
If so, the copy idea would be the way to go. It would be pretty dumb if all slots were reset on a client switch, so I sure hope it is not working that way.