DXS is time based (accordingly to the prime 1 site). Their approach and wording allows them to kill two birds with one stone.
I.E suppose the 999 DXS didn't sell well, the remaining 999 will convert to the DX version after a set period of time (hence the time based description). Or the 999 DXS sell out and everyone makes it aboard on time, everyone of the 999 is a DXS. Regardless of whichever scenario plays out, the ES will remain at 999. IMO this approach is reasonable and is what most collectors (including myself) expect.
Alternatively if it sells very well and the entire 999 DXS is allotted, they can decide to segment the DX version in the near future with a different ES and state the time based window for DXS has 'expired'. I personally don't think they will go with this second approach as it's sneaky and shady. They've also got the regular edition to milk any remainder buyers who missed out on the DXS. However, nothing is set in stone, it's Prime 1, from this statue we've learnt to expect the unexpected
