Hi there, just to share what i have gone through.
My unit just went through the HIP 2 months back and the HDB contractor informed that they will do a hack through when they change the piping, i.e. including removing the cast iron ''ring'' that is embedded in the ceilings between both units unless your neighbour above or below your unit opt out for the upgrading, then they are unable to perform the hack through but just cut and change the portion that can be seen in our kitchen.
For my case, my piping from the top was a cut through but bottom wasn't as unit below mine opt out for the upgrading.