imo
1st gen
officially won't possibly see an update to wp8 since it already got 7.5 unless ms states otherwise... As for the dev community seeing as i would assume all 1st gen are already hacked (hspl) it's only a matter of time
2nd gen will see an official update to wp8 since they came out with 7.5 and by the time they announce wp8 they are only like a year or so old while 1st are 2years.
Basically
i would think that microsoft has a 1step update... Your phone will be updated one version up from what it was released with.
The only main hardware difference from 1st to 2nd gen was the camera and faster clock speed.. Sure theres more underlying differences but those are the two notable changes.
Wp8 will integrate the camera deeply with skype/wlive and maybe a face unlock (doubt it).
But the thing about wp7 is its simplicity so theres no reason why all wp devices shouldn't get an update to future version 9+ (carriers will stop them for new profit). Right now the only thing that would require real processing power are games and maybe a possible form of desktop/console integration.
Or maybe a new soc from nvidia or ti that would have a big architectural change, but then again i would think they wouldn't throw qualcom in the fire.
I like speculations