I am in a similar situation in that I booked flights over 9 months prior to our trip and our flights were rescheduled and all screwed up. Not only did they cancel our original flights completely, but when they automatically rebooked us, they put us on flights with 9+ hour layovers each way. I am travelling with 2 kids under 6 and a disabled family member. That would be a nightmare for everyone involved. I finally got our flights worked out, but could not get seats together, plus had to add an extra layover coming home. Our original 7 hour journey will be 13+ hours coming home. Not really looking forward to it.
One of our flights is completely sold out, and my disabled mother does not have a seat assignment. When I rebooked the flights, they told me she would be accomodated at the airport (at that time there were a few seats remaining). I have called three times to give the airline an opportunity to move people around prior to arriving at the airport, but they will not do it, and now coach is completely sold out. So yes, someone is going to have to be moved in order for me to sit next to my mom to assist her in sitting and standing during a 4+ hour flight. We have less than a 45 minute layover during which this seat assingment is going to have to be made, so it will be interesting what the airline does to accomodate her. I'm hoping she gets bumped to first class, since the only 2 remaining seats are there.
So, I feel your pain, but it could be worse. If you do have to be moved around to accomodate someone else, be glad you are not traveling with small kids or the disabled, for whom it is (in most people's minds) unreasonable to be away from their traveling companions.
I hope you are able to work out seat assignments that work better for your group, and that you have a great trip.
I feel a little better after venting! (Keeping my fingers crossed for a kind soul to give up his or her seat next to me on our sold out flight, or for a compassionate flight crew to bump mom to first class)