Gee is quite right, Topsec, when she says that the fact you were your Boss's PA in a previous life is irrelevant.  However, it is still important.  

There are good things about knowing your Boss already - you know how he works, how he thinks and can anticipate his requirements.  The downside is that you may know too much at a time when he has to develop his post and he may have new requirements.

Eight years in a post is a long time, and adjusting to a new post with all that that entails won't necessarily be that easy however flexible and adaptable you may be.  It's the same for your Boss, too.  

A newly-created post in a new company is another thing as well.  You will just have to make it up as you go along and develop the job pro-actively.  

People are often a little wary of staff who are "imported" from manager's previous jobs, and sometimes you may have to contend with people who applied for your new job but were unsuccessful.

So, I would advise you to approach your job as you would any other new job with enthusiasm and creativity, but tread softly with your new colleagues.