I know that the pope lifted the excommunication, but I was not aware that Williamson is actually recognized as a bishop by the Church.
His episcopal consecration, like that of the other three bishops consecrated by Archbishop Lefebvre, is “valid, but illicit.” In other words, he is a real bishop by virtue of the fact that Archbishop Lefebvre, undoubtedly consecrated validly himself, consecrated him among the four. However, he did so without the pope’s approval, and their consecration was therefore illicit, or “unlawful.” In the event that any or all of these bishops return to Rome, they will not have to be “reconsecrated.” They will, however, have to make some sort of encompassing profession of the Catholic Faith, insofar as they were part of a movement that has self-identified as being outside of Rome’s jurisdiction and has declared the (authentic) Catholic Church to have accreted to itself various beliefs not found in, or substantiated by, apostolic Tradition.