Firstly, tbh mill design etc is not at all my comfort zone I’m...

Firstly, tbh mill design etc is not at all my comfort zone I’m certainly not in the know in all the options and limitations, my experience is mostly second-hand working alongside them.
But what you raise is exactly what I have been wondering about…and the short answer is absolutely it could, but at what cost.

To put some context to that statement, from the information it seems like HAS have settled on a plant design for 1 million tonnes per annum throughput rate, with a rare earth concentrate output capacity of 35,000 tonnes per annum. Now obviously that’s all optimised for the Yangi mineral resource…and we can safely assume they won’t be building in extra capacity on DREs behalf.
But in many instances, particularly if there is potential for additional resources etc then the mill might be pre-built with some additional capacity or at least designed (layout, spacing etc) in a way that considers an eventual upgrade.
Without knowing the exact specifications of the different components are and having someone to ask those questions of..its difficult to know if what HAS are planning, in its current form could take additional capacity…I suspect ‘some’ additional capacity would be possible. Then there are other improvements like LoadIQ to optimise mill loading (up to 5% increases in throughput).
The other option if throughput is a limiting factor is to instead focus on optimising the feed material, including things like XRT ore sorting (which HAS are planning on) and potentially by blending high quality ore with lower quality to keep the throughput rate the same but produce a higher quality concentrate at the back end.
But also, just for argument sake like you mentioned, if a regional consolidation was to occur and a situation developed where a pre-existing 1Mtpa plant needed to be 1.5 or 2Mtpa then large scale mill expansions are not out of the question....then its just a case of does it stack up economically for whatever that consolidation consists of vs the other options above, which is a question that would need to be addressed before anyone made a move. Obviously to answer any of that, need to know what is in the ground for DRE....so keep on drilling.