Oh, I do understand how you feel. I've been struggling with a similar but less drastic situation in my family. It's had me in tears and feeling panicky inside.
Firstly, I'm with you on retiring to the US. I wouldn't want to either, especially with having a nice home and living in an area you love.
I have said to my DS that I'd rather he was in loving contact and on the other side of the world than round the corner and not speaking. Since your DD has suggested that you go to California, it sounds like she thinks she will miss you, too - which is good!!
Don't panic about it. They might not like it. If they do, they will, no doubt, come home to visit and you will be able to take some fantastic holidays. Or they might be happy there for a while but later the situation could change. In the meantime make full use of all the communication opportunites that modern technology allows.
Like me, you probably will still get upset from time to time but it might not turn out nearly as bad as you fear. In fact, your fear and imagination might be making you feel worse. Try to take a deep breath and go for the 'wait and see how it turns out' option.