If I read it right, no problem. Keep in mind, you can NOT two different companion tickets alive at the same time. Pretty sure that once you drop wife and add daughter as companion, ticket for wife goes away.

Not sure why you would WAIT to do the switch. Do it now and book daughter now. Once onboard (or maybe after landing), add wife back and book her.

Only possible downside is that there are no seats left for wife. But based on many stories of folks (like me) that forget to add companion and find a flight is sold out.. .keep checking and a seat opens up. You could also buy a paid ticket for wife now (I would do points) and then cancel when you get her added as companion.