Westy's GU

[the evil twin]

GCM will most likely be the same as original.
None of the mobs I am aware of do GCM upgrades (but that is only a couple)
GCM upgrades are available but if they are like GVM upgrades in some states they must be done at time of Rego
Lot more engineering involved in changing GCM

Disclaimer, Some states do not apply GCM to light commercials only LR, MR, HR etc sooooo it depends what data is supplied by the manuf but you still can't exceed the GVM upgrade specs.
Some companies specify that GVM upgrade is against GCM some do not.

Should have added irrespective of the vehicle IF the GCM is stated by the manuf on the vehicle plating then that is applicable
If it is not then the GCM is 'as determined by the registering authority' IE the dudes on the side of the road or at the inspection station.

It's a huge bag of regs and bulletins etc but easiest way is stay under the original GCM (if plated) or 'original GVM + towing capacity = GCM even with upgrade' and you are def OK.
Trying the 'updated GVM + original towing capacity is now the new GCM' may or may not work if things go pear shaped