I am interested in knowing, on average, how often the professional
labor force (making over 100k/yr) is changing employers.

More specifically how this data applies to patent attorneys.  That is
to say if there are 30,000 registered patent attorneys how many, in
one year, are changing employers (not changing careers).

To answer your question outright I can not be sure, but my
father-in-law is a Senior Financial Planner and also VP for a leading
bank chain in the NJ area. He makes well over 500k a year and has
changed jobs 5 times in the last 4 years - better jobs - better pay. 
In this era there is always more money to be made somewhere else it
seems and the better you are at what you do the higher ranking, higher
paying job offers come alot more frequently nowaday's then they did
say 10-15 years ago.