You are very fortunate that the dodgy by-laws have been found to be defective. It really doesn’t matter what the chairman or his cronies want – there is a legally established formula for deciding theses things and procedures available at NCAT for forcing the committee to do the right thing if they choose not to.
In very simple terms, the people benefitting from the attic conversions have to pay the Owners Corporation a sum roughly equivalent to the increase in value to their homes, minus the reasonable costs related to the conversions.
The same applies to the basement space being demanded in return. Even if a majority decide to ignore this, it’s called a fraud against the minority and it can and should be reversed.
You need to get a neutral assessor (a real estate agent will do it) to work out the relative values.
If the self-interested owners upstairs refuse, tell them that not only will you take them to NCAT to force them to do the right thing, you will also insist that the Owners Corp charges them rent for the years that they have illegally been using the space.