It’s unlikely a bylaw transferring Windows to owner property would be legal or upheld by NCAT unless all owners agreed. That means all 4 owners , not just the usual 75%, would have to agree to the bylaw.
If the windows of the affected lot owners are in a state of disrepair, then they must be fixed. That’s the obligation of the OC. If there are insufficient funds, then a special levy needs to be raised. It’s no defence of the OC to claim lack of money.
But it seems to me that two owners want the Windows replaced largely on aesthetic grounds. In that case the owners can pay for it themselves.
A bylaw may not be necessary, if the OC decides to own and maintain the Windows in the future. The other owners and the OC are getting replacement windows at no cost, putting off a cost of the future. And if the Windows are installed correctly, the maintenance in the medium term will be less than if they were not replaced.
The owners paying for the Windows my feel aggrieved in the future if the other Windows are replaced by the OC at the OC cost.
Final word. An owner does not own the common property. An owner may be dissatisfied with the style, colour or aesthetic of the common property but one bought into the strata on an “as it is” basis. To expect that the common property can be changed to suit their tastes is folly.