After a lot of thought and research, I decided to replace the old roof with Vermont Semi-Weathering Gray slate.  Some tiles will turn buff or brown, which should result in an appearance similar to the original. 

The plan is to remove the old slate and then tackle the gutters and any other structural work that needs doing before the new tiles are installed; then the crew can move right along once they start with the replacement tiles.  The slate is scheduled to be delivered in a week.

The crew (foreman and three workers) arrived about 8:30 and set up; they kept going until 7:30 PM.  A dumpster was dropped off at 11:30. 

By mid-afternoon they had removed all the old slate.  They bagged up the remains of the old black underlayment and swept the deck boards with a broom (lots of gray dust).  Finally, they removed the old metal valleys and ridge caps. The removal took less time than I thought it would. 

Then they installed a waterproof membrance.  Finally they covered the roof with the usual blue tarps for the night. 

The dumpster (right-click / View Image for larger):

Old slate coming off on the east side (right-click / View Image for larger):

Old slate removal on the west side (right-click / View Image for larger):

Old slate in the dumpster (right-click / View Image for larger):

Installing the waterproof membane (right-click / View Image for larger):