If he really admitted the crime and pleaded guilty, his status as a Thief in Law can be taken from him. One of the main rules to become a Vor V Zakone is to never admit your crimes. However, I do not have information if this rule has been changed in present day like we see that Thieves in Law have families and properties, which was also against the rules in the past.
I've also read that the Armenian Thief in Law Armen "Pzo" Kazaryan has pleaded guilty in trials before. But there is no information of him being de-crowned by other Thieves in Law. Maybe just because he is all the time in USA.