Lawof the Land – In my opinion the short answer is ‘Yes’. The change of material is an improvement rather than a repair.
If it is not already too late for you to do this and the additional expense of the colour bond slatted material is particularly concerning to you then you can submit a Motion yourself at the next AGM with quotes for repair of the existing material, repaint, and re-installation of the wooden lattice fencing back to its original position. This is assuming that the existing fencing material has not fallen into such disrepair that it is irreparable.
You could ask your Strata Manager to help you draft such a Motion. There are tips to writing Motions available on the internet if the Strata manager is unhelpful.
Usually the EC obtains quotes for Common Property repairs but being that they are probably on the side of the ‘fence replacer’ then they would probably not be interested in obtaining quotes for a simple repair. But there is nothing preventing you from obtaining quotes for this work.
Anyone who is entitled to vote at a general meeting can ask for a motion to be put to a general meeting. Written notice must be given to the secretary who must put the motion on the agenda for the next general meeting. The written notice must:
(1) set out the motion, and (2) name the owner who made it, and (3) have an explanatory note of up to 300 words.
The OC can decide your Motion by a simple majority at the next AGM.
The same suggested process to obtain quotes could be worked through for the front gates. I would probably treat the rear courtyard and the front gates as two separate Motion items to enable each to ‘stand’ or ‘fall’ independently.
Your Motions may be defeated at the AGM and the OC may decide to proceed with the more expensive ‘improvement’ but at least you would have tried.
If you are too late to submit a Motion and the Agenda has already been set then the chairperson can rule a motion as “out of order” at the General Meeting itself if the motion conflicts with the Act, is unlawful or unenforceable. A Motion that does not clearly state that a special resolution is required would probably be unlawful. You can raise your concerns at the AGM itself or prior to it.
See here for a summary of The Owners Strata Plan 50276 v Thoo [2013] NSWCA 270 that you might find helpful and interesting. A full reading of the actual case would be even more so:
https://www.cbp.com.au/insights/2013/october/owners-corporation-has-duty-to-maintain-and-repair