Obligation to bring carports up to standard

I think the principles in the ACT are the same as NSW and elsewhere. In general there is no obligation to bring a structure up to the current code if it is safe and well-maintained. On the other hand, if there are substantial repairs required, not just simple maintenance, then the new work would need to meet the current standards. Eg. you might have a balcony where the balustrade met the standards at the time of construction. If you are just doing maintenance, say replacing a few pieces of timber or repainting, it is OK so long as it continues to meet the old standard. However, if you need to make more major repairs, more like rebuilding, then you need to meet the current standard. 

As it happens, our OC had a situation a few years ago where two private carports on class B unit subsidiaries had brick walls that were bowing and cracked to the extent that they were unsafe and could collapse, which would bring down the roof and could have killed someone. The cause was that the walls were acting as retaining walls for soil on the adjacent common property. While the walls had been correctly constructed as retaining walls, the common property (reactive clay) soil had been replaced without providing subsoil drainage and the depth of that common property soil had been allowed to increase over years to a greater depth than the walls had been designed to retain. 

The OC agreed that the damage to the private property was caused by a defect of the common property and so it was an OC responsibility to rebuilt those carport walls and to do so to current standards. 

Anyway, back to your carports. What prompted getting an engineer’s report in the first place? If the carports were built to the standard relevant at the time of construction and have been maintained to that standard and if they are not failing in any way, then I would say there is nothing to be done. 

If they are failing in some way and need more than simple maintenance to be made safe and/or just not fall down, then the new work would have meet current standards. The easy answer from an engineer is, of course, to say build a new structure to current standards. Another engineer might well be able to advise some remedial work that would be sufficient as you seem to suggest. If a modification would mitigate some safety risk and could be easily made then you should do it in any case. 

We had a similar situation with playgrounds. Even though it seemed that relatively few things needed fixing, nobody we approached was interested in a solution that would fix our old but well-loved mainly-timber equipment. They just wanted to demolish and replace with boring equipment like the kids would experience anywhere else. We were lucky to find one person who was appropriately qualified to restore and modify our equipment to meet current standards.