Just a quick summary of your position.
Changes to common property require a special resolution (75% in favour), repairs require only a simple majority.
If the pro-work group are saying that the current balustrades need to be changed because they aren’t up to code, then that is an alteration of common property that requires a special resolution (75% vote in favour).
If the pro-work group are saying this is only a repair, requiring a 51% vote, they have to show that the balustrades are broken (which they are presumably not).
There are a limited number of instances where building codes are retrospectively enforceable (fire ordinances and swimming pool fences, for instance). Most other building code issues only become enforceable where there is work being done in the area where the non-code structures exist.
If they dispute this, and they want legal advice, they do have a majority so they can force everyone to pay for it but you might have recourse under Section 149 of the Act to ask that levies be altered so that those who did not see the need for legal advice don’t have to pay for it (especially if they are found to be correct). I would certainly present that as your intention if they insist on the owners corp paying for further legal advice in a matter that is already being considered by NCAT.
The simplest solution would be to let the upgraders upgrade their own balustrades at their own expense, with the condition that the same will apply to the other owners should they wish to do so, and that the individual owners accept responsibility for the repair and maintenance of the balaustrades in perpetuity. The cost per owner will work out roughly the same without the need for lawyers (except to draw up the requisite by-law).