If it's only the storage subsystem, bundled games, controllers, etc. then that isn't even anything notable. You can do that mid generation just like with past consoles.
Anything that potentially impacts game performance however will be a non-starter. Unless you don't plan to launch with any games. To hit the launch window a game would have had to have been in developement for at least the past 1 year (small budget limited scale game) to 2+ years (large scale AAA budget title).
Hence, anything of any significance will likely have been set in stone at least 1 year prior to launch. Hence, why it is likely that both consoles are likely using Southern Islands based GPUs rather than anything newer.
I suppose it may be possible to bumb up memory capacity or speed of CPU/GPU without impacting ongoing game developement, but I find that unlikely as that also means redesigning the PCB and requalifications in most cases. Not only for the PCB, but for the case, power supply, cooling solutions, etc.
Regards,
SB