In my experience you always loose employees when moving offices. It's a really weird feeling just after the move and takes a while to get back into the grove.
Up in SF, moving the office one mile can add 30 minutes to some people's daily commutes each way.
Meanwhile, moving from one office to another within Sunnyvale won't change much considering there are many ways to get in and out of there.
Moving from one city to another can be highly disruptive.
I was trying to stick it out and wait for the culture and mood (which had been fantastic beforehand) to get back in the groove, but our best people started to leave one by one, then one of the founders of my current company reached out to me personally, and it was too hard to pass up.