Thank you SHR,
That was my original plan, out of fear of damaging the stack.
The rep at a the big colored box store said take it all out, right to the wye, the cast is holding steady at 68 years of age and I don't want to temp fate too much.
This is the last few feet of galvanized having pulled about 28 feet out over the last year.
I will stick with your plan of attack.