clarifications in case anyone wanted to know:
can i still have sero? - yes, but the catch is that you need to have someone with a sero plan add you to their account, then have sprint replicate their old sero plan onto your number, then do a TOL (transfer of line) so you can have your own account and not be under theirs. it takes a while, but its definitely doable and worth it. they will eventually stop this though, because sprint knows that people are doing this to pay less...
can you still add a line with a $30 or $50 sero on it? - yes! as long as you have an old sero plan in your account, sprint is legally required to give you the plans that you have to another line that you open.
can you get any phone on the old sero plans? - yes, except the instinct and instinct s30. you can get the blackberries on the old sero plans, but they also require the blackberry data add-on, and unfortunately there isnt any way around that.
can you change plans without extending your contract? - yes! sprint finally started doing this a while back, but just wanted to tell everyone in case they were on the fence.
will the pre work with the old sero plans? - who knows...but unfortunately, its looking less likely at this point. sprint wants to treat this phone like the instinct, so that means you need either the simply everything plans, the new sero, everything plan with data, or some business essentials thing.
why does the pre need the new plans, especially because i dont want to pay more when i already have unlimited data/texting, etc. - well i can think of only two things. one is because sprint is losing money like no other, as well as customers, and sprint wants them back, and by giving customers a cheap all-in-one plan, they get the best of both worlds - a "cool" new phone and a "cool" new plan. secondly, with the old plans, you could manually add on and take off data, because sprint had marketed data like that. however, with the new sero and the new simply everything plans, data is now part of those plans and absolutely cant be manually removed - this way, sprint wont get hassled by overage and huge data/texting charges, and the customer remains "happy". this is why the instinct required the new plans, cause it has visual voicemail and wi-fi was deliberately left out (reminds me of verizon), so they can use data.
and who knows, sprint finally wants to get rid of the last remaining $30/$50 sero people so they can get more money. by restricting certain phones on the old plans, they think it'll force them to upgrade. yeah right, definitely not me.
hope that helps!