paintbrush.pgm and paintbrush01.pgm are binary identical. If they were
brushes we ship as data, that would be more annoying because they could
be used in scripts or in various other places. But here it looks they
are used for gimpressionist plug-in only, which means they are probably
not used in some random script. All it takes is probably only to
properly migrate gimpressionist presets.
Presets using "paintbrush.pgm" will be migrated to use
"paintbrush01.pgm" instead.