I mean:
-> e.g. you ordered and payed for a Saturn on 20th June
-> Now you upgrade to Jupiter lets say on 10th August - you also do your "upgrade" payment on 10th August
What is the "paying" date for your order? 20th June oder 10th August? As far as I understood this will have big impact on delivery date of your machine due KnC said "First paid, first serve".
I think the order/payment position of your Saturn remains, and the new order for the upgrade gets a new position.
Meh w.r.t. production you're talking of a module or two added to an existing order, not a brand new order of multiple units. No one will have ordered multiple Saturns when they could pay less for Jupiters and an odd Saturn at the end if that's what they could afford at the time. So even in that example at worst it's an additional two modules to an order...
How would you add modules to an existing order? That's not possible in their shop.