There have been press releases online saying that you will not be able to get the Pre with the old Fair and Flex SERO plans. They have said you will need "an Everything Plan." So what this leaves open is that you may be able to get someone to upgrade you to an Everything Plus plan and then be under the "Everything Plan" umbrella. The easiest way I see is to just log into the 'SERO' page the day the Pre comes out and see if that phone is avail. Or you can call like I probably will, since I'm out of contract, and see what you can get done.
Another note: on people complaining about Sprint trying to get people out of the SERO plans by blocking them out, my opinion is you have no room to complain that much. Sure it sucks for me too that I may not be able to get the Pre without giving it up, but you do realize that you rape sprint every month you have that plan? No other plan on any other carrier comes close to that price. So despite agreeing it sucks, your complaints that Sprint is a horrible company ect. are ridiculous. SERO blew up on the online community and Sprint is doing everything they can to actually make a couple dollars off you, instead of basically none. Crazy concept huh? ;P