Its hard to say in this instance with the NBN being so new whether any provider has asked for further upgrades yet. Under the old system some of the more popular ISPs such as Internode/iiNet/TPG etc were consistently asking Telstra/Optus for more space in the DSLAM. Sometimes this was forthcoming and other times not. Often its decided based on whether there was any financial incentive for those upgrades to be completed.
One could assume given that NBN Co. at this moment is government owned, there would be less of a need for a “business proposal” surrounding the need for further upgrades (this is practically the reason why the NBN exists as it does today anyway). The priority is and was underserviced regional areas because there was no way Telstra would make any money off further upgrades.
Now whether this model continues to occur under NBN Co. once the rollout phase finishes is a little hard to tell considering the relative “youth” of the NBN Co. But I would assume there would at least be “provisions” for further upgrades at a later date.
With fibre to the node, it’s quite simple, in some cases you don’t even need to replace the cable to enable more speed. This was one of the core reasons for going with fibre (its upgrade potential). Copper on the other hand? They’re still struggling with getting around a gigabit line speed over more than a couple hundred meters. That’s the bigger issue.