All I can say is make sure you know who you are contracting with. The finished product might look ok from the ground, but I've seen roofers skipping the water and ice shield in the valley and weaving the under layment up the valley. I've seen them split the water and ice shield from three foot to 18 inches to make it stretch per roll and the underlayment not reach the valley water and ice sheild. I've seen gaps in the under layment where it didn't over lap the lower course when running past a perpendicular roof . I've seen where the new under layment stops a foot from the ridge. In all this the old underlayment was left on so hopefully it works out. So make sure to keep your contractor honest and do a proper job . Watch them work and ask questions. Goes for the siding and facia finishes too.