Considering their 230M POP goal of 1H2014, I don't think it is completely inconceivable for them to make that goal. That covers their existing UMTS footprint.
As for reaching 250M by end of 2014, I have my reservations. Thats a ton of sites to get that extra 20M covered. I won't hold my breath, but I do think they will start drawing their way out of the "ink blots" and letting the pen bleed a bit. A fair amount of that old GSM only equipment isn't much longer for this world. It has a finite lifespan and its likely nudging its expiration date. I wouldn't be too surprised to see some of those sites make the jump first.
As for what they are upgrading to/from, a rural site is likely at most broadcasting 2 or 3 GSM channels with GPRS/EDGE services in those time slots. So thats at most 600 kHz FDD per site assuming a 3 sector rural site, which there are plenty of 2 sector sites out there on highways. It takes very little frequency reuse to keep GSM chugging along on those highways, realistically 2 MHz FDD or less should be plenty to cover a highway stretch with frequency reuse I would think.. I would expect a minimum of an AWS and/or PCS HSPA+ carrier and a 5+5 LTE carrier in AWS spectrum allowing. They have at least 10 MHz of AWS nationwide, so there will be some type of AWS service living there, and my bet is on LTE where they are constrained and both UMTS and LTE where possible. So at most you would have GSM1900, UMTS Band 2 and 4, and LTE Band 4 and 12. Doubt we will see rural Band 2 LTE except possibly on old MetroPCS markets where they didn't have AWS like Atlanta.
As a T-Mobile subscriber, I obviously have an optimistic slant, but I also view this realistically and with a grain of kosher salt. I want Sprint to succeed, I want T-Mobile to succeed. I think both of the 3 & 4 carriers doing well will shake up the duopoly a bit and help even the playing field a bit. I just don't want the anti Sprint label placed on me for being in the Magenta camp (I have a FreedomPop hotspot for EvDO on car rides if thats a sign of good faith).