70 year old Cast Iron Boot Vent can it be reused

I am replacing extra thin Cast iron pipe with PVC. its full of pinholes and cracks.

The old vent pipe is 4 inch Cast Iron ; the Boot is made of cast iron;
and the 4 inch pipe is just pushed through and sealed with tar. That's what it looks like
the vent is probably blocked by bird droppings. We had a sever thunderstorm last night and no water dripped down the vent.
Can I pull the cast iron out and slide the PVC through and just reseal it with Tar ?
I heard that PVC is Sun light sensitive; can the 18 inches poking through be damaged by the sun ?
I will screen vent cap the new vent to keep the birds off of it.
I am dizzy; and don't want to climb up the ladder any more than I have to.

I do not see anything that makes me think the boot is "cast iron" and IF it were it would be the only one I have ever seen. Usually they were lead around a cast iron pipe but it could be tin with tar. You put a coat of paint on the exposed part of the vent. IF you are "dizzy; and don't want to climb up the ladder any more than I have to", do NOT climb up on the ladder at all.