When vias are created, they are created with the "default" drill size or the current "alternate" drill size.
The default drill size is recorded in each individual via as a magical value of -1. The current alternate drill size is whatever the current mil or mm setting is for the "Alternate Drill" size in the Dimensions | Tracks and Vias dialog. That value is used in place of the -1 in each such via. The interpretation of any AND ALL vias subject to the "default" via drill size can be instantaneously changed merely by changing the default drill size in the same dialog. You do not need to visit each default drill sized via. Only vias which have an alternate drill size setting need to be revisited individually. And of course, as always, you can edit these Kicad files with a text editor, but the format is not self evident so always keep a backup before you begin experimenting with a text editor. The file formats are documented elsewhere however. Dick
