I guess it depends on what you wish to do. Career change to teaching will need a PhD. Career change to different industries will need an MBA.
So if career change for an experienced professional like you is really towards teaching, and you do not mind a hard grind and at least six more years (one and a half year before you get in anywhere since you will aim for fall 2007 entry - and then at least four years before a degree) then - AND ONLY THEN - you should think of a Phd.
Please do not think I am discouraging you. I am probably the same age as yourself . But I do wish to emphasize that at this age, this decision is a big one. There is no bravado involved here, just your own inner voice. I am enjoying my life in academia - so far. But I do know of many people who have left the PhD program - the dropout rates are close to 50% at many places, because they realize that PhD is not for them. Now that is one experiment we oldies cannot conduct
Hope the above is useful. Please post again if you wish, and I and others can discuss further.
On the other hand, an MBA is fine for a career change. One year MBA in Europe from IMD or INSEAD is great value, poses much less disruption and will surely boost the career. It is not cheap though !