Pb2+ can act as a network former in oxide glasses when its lone electron pair is sterically active such that the Pb2+ ion forms only 3 to 5 short, strong, bonds to oxygen, typically in the hemisphere opposite that occupied by the lone pair.

By the way, I would not say that there is clear evidence for a strong change in behavior of Pb2+ between modifier and former in pure PbO-SiO2 glasses. Such a transition is much more clear in other systems such as PbO-B2O3.

So no, the modifier-former role of Pb is typically not related to oxidation state changes, rather it is the steric activity of the Pb2+ electron lone pair, or similarly the polarization of the Pb2+.

Adding other elements such as alkali to PbO-SiO2 can have a big impact on the role of Pb, because it can encourage formation of more ionic Pb-O bonding (longer weaker bonds) which cause less polarization, and more of a modifying role for Pb2+. This last part is my personal understanding/conjecture.

Please see:

Thesis The Structure of Vitreous Binary Oxides: Silicate, Germanate...

Article Lone-pair distribution and plumbite network formation in hig...

Article On the germanium–oxygen coordination number in lead germanate glasses