Thanks in advance, Jeff R.

--- Other Info ---
We are in the Midwest.
Believe the existing shingles are asphalt organic felt (does this matter?

No.

Some of the existing shingles have started to curl slightly.

Should be ok if you choose a dimensional(cedar look) shingle.

Guys putting up cheap shingles must do a tear off. Poor quality
shingles look awful going over anything but sheathing. We put 30 year
shingles over decent roofs all the time. It's called a "roof over" and
exact directions are on every bundle sold. 99% of the country allows
upto three layers of shingles with some regions allowing four.

The second part of the scam is that the lowest bidder saying a tear
off is needed, always "happens" to find a lot of damage. Possibly
charging $100 or more a sheet to replace the sheathing. Only when
everything is said and done do you realize they were actually the
highest bidder.