  In this paper we present the current status of a XMM-Newton program to
observe an optically-defined, complete and unbiased sample of Compton-thick
Seyfert 2 galaxies. The main goal of this project is the measurement of the
occurrence rate of transition between transmission- (i.e.: Compton-thin), and
reflection-dominated spectral states. These transitions potentially provide
information on the distribution of the obscuring matter surrounding the
nucleus, and on the duty-cycle of the AGN activity. With about 2/3 of the whole
sample being observed, we detected 1 further transition out of 8 observed
objects, confirming previous suggestions that these transitions occur on
time-scales ~50-100 years.
