We just had our roof torn off and re-shingled. The old
Ice & Water shield was well bonded to the plywood and
cannot be removed. They simply installed another layer
bonded over the first layer to achieve the seal needed.

Your old I/W shield probably had a smooth top surface.

The kind they use today have a granular top surface.

Does anyone have any observations of roofers laying new I/W shield
over top of old GRANULAR I/W ???

Again, I think that both the sticky underside and the granular top
side of these membranes is only for product liability. Any roofer
walking on a properly applied granular-top-coated I/W membrane will
have plenty of grip.

That's nice if you make these membranes, and I'm sure it's nice for
roofers working on the high-sloped mansions that are all the rage now
aday's, but it means SQUAT for the low-slope home owner that has to
deal with tearing the hell out of his decking in 20 years because of
friggin sticky-back membrane. (!)