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In this paper we present the current status of a XMM-Newton program toobserve an optically-defined, complete and unbiased sample of Compton-thickSeyfert 2 galaxies. The main goal of this project is the measurement of theoccurrence rate of transition between transmission- (i.e.: Compton-thin), andreflection-dominated spectral states. These transitions potentially provideinformation on the distribution of the obscuring matter surrounding thenucleus, and on the duty-cycle of the AGN activity. With about 2/3 of the wholesample being observed, we detected 1 further transition out of 8 observedobjects, confirming previous suggestions that these transitions occur ontime-scales ~50-100 years.Comment: 4 Latex pages, 5 figures, To appear in the proceedings of the  Conference "Multiwavelength AGN Surveys", Mujica R. and Maiolino R. Ed

TRANSMISSION- TO REFLECTION-DOMINATED TRANSITION IN SEYFERT 2 GALAXIES: THE XMM-NEWTON VIEW