"...Career wise we NEED to do it..."? I know this isn't the question - but I'll say it then go on to your question. There is never a NEED to move across country and away from all of the people who are important to you unless you have no job, no income and no hope of finding it where you live. Kids need to have other people around who love them - grandparents, aunts, uncles, cousins, etc. I doubt you can change your husband's mind now - but I'd try. I may be presumptuous, but to me it sounds like your husband is very career oriented and wants to move up the ladder. That's completely understandable - and it may be a great honor and compliemnt for him to accept this job in Boston - even if it means you and your daughter will be isolated, in January, in Boston. On the other hand, if this job is neccessary becuase his company is closing, or his job is being eliminated - then you really have no choice.
Regardless of the underlying situation, if you're going to Boston, you're going to Boston. Put on your big girl dress, smile and make the most of it. Get some travel books out of the library about Boston. Find some good websites about Boston and tell your daughter sooner rather than later. Be realistic about the sadness of leaving friends and family - you can't really sugarcoat that - but you can present the move in the best light possible. Tell them yes. we're going to miss Sally and grandman, but we're going to do these things to stay close to them:
If you can, travel to Boston with your duaghter for a weekend with her before the move - take her to all the cool places in Boston - the children's museum, on the Ducks tour, the USS Constitution, Fanueil Hall, Qunicy Market, etc. If her best friend's parents would allow it, take her best friend so they can see it together.
Get set up with skype on your computer and with your family in CA so you can all video conference online. It's not perfect but it's better than nothing. I know lots of family's who stay in touch this way - even around the world.
Fortunatley, Boston is a good vacation destination - so the possibility exists for lots of family visits. Encourage that - make sure you get a house with a spare bedroom, etc and send out constant invites to your west coast friends and family.
Just make sure to be upbeat but realistic. Tell her you know how she feels becuase you're sad about missing your friends and family too - but you're also excited about this new chapter in your family's life. Afterall - it is exciting too. (My family moved to San Diego from NY when I was 8 for a year - and it was fun, exciting, adventurous - but we knew it was just for a year) Tell her that you'll now be able to go to cool places that are tough to get to now - like NY, Washington DC, Montreal, etc.
Finally, at age 9 kids are still pretty flexible. She will probably makes new friends at her new school pretty quickly - probably more quickly than you will. (sorry to say). Age 10-11 is probably when it gets really difficult to move so it's going to be OK.
When you get there begin to get plugged in right away - find a church, lots of them have MOPS (Mothers of pre-schoolers) groups so you can meet other moms with kids your son's age - and you'll find that some will have daughters who are 9. On the cold winter weekends in Boston go to museums, learn about the city. It's a small, walkable, college town that's rich in American history. You will have fun and it will be an adventurous time in your family's life.
Good luck mama!