Not really an upgrade.4/6 mtpa is a new plant in a different...

Not really an upgrade.
4/6 mtpa is a new plant in a different location, specifically for the bulk tonnage of DSS

In time the existing 450 (which I read has a 500 capacity) may be upgraded to 1/1.5.
So 2 different items.

Main question is the feasibility of early start up and the how it all works to generate revenue ie. sell the concentrate to who? and by what means?
Export tariffs! I don't see why BSX would be deserved of a concession from VN government as some has suggested, sets a precedent and shows favoritism, would be nice, but if you were the VN treasurer would you like to see a lose of revenue and bending of rules to suit 1 coy?

Therefore to sell concentrate at 85% market price and incur further tariffs, we lose the opportunity to process that material and gain 35% premium and remove tariff payment. there is a opportunity loss in this scenario and are the gains (raising capex) justified in the loss, when a further raise at higher SP and debt funding may provide the better alternative, sure management will be working this all out.

That is the problem the market has with BSX and why the SP struggles to hit ath again, as a whole its an awesome and rewarding project, but there are still many unknown factors and no clear set pathway has been forward.