But you are not changing the actual page type definition, right? The user is changing some actual value in one of the pages, represented by given page types. Just to be sure, the user is not adding any fields to the page type, is this correct?
If yes, the user is editing the page object, not the page type. The page type is just the data structure container and the event is not fired.
So, as Rui suggested you need to use the document (pages) event handler in this case and do the check on the NodeClassID to check if given updated page is matching your page type.