Does 'FALLING APART' mean that the shingles are hard and brittle? For instance, a nail will crack the shingles instead of drive through.
Yes, that is exactly what I mean
Because if you do install the new over the old and the old ones crack and fall apart, they will crumble away and then leave a gap between the new and old materials.
then the next time you get up to walk on the roof, the nails will poke through the new materials creating leaks.
And the wind will also be able to get under the roof and tear off the new materials