The old 110 volt air-conditioning unit had to go from the roof, but the 3 existing roof vents had been replaced at some time in the past and looked in good condition, so I didn't bother with them.
However, once the internal roof lining and insulation were out, it revealed an issue with water leaks from the roof vents and pipe caps
These leaks were caused by stupidity.
When the roof vents had been replaced, they hadn't bothered to remove the original 'putty' sealant ... just smothered everything with silicone sealant and refitted the new vents. Problem is that the two materials don't bond together, and over time water found it's way between them.
If you're replacing any item which has a water seal, STRIP & CLEAN ALL SURFACES PROPERLY otherwise you are wasting your time & effort!