When can/will Sprint unilaterally change your rate plan?

[quick99si]

I remember being told by AT&T (before they were Cingular before they were ATT) to get a new phone+plan or GTFO. I believe it was during their "all-digital" conversion way back when. I spoke with my wallet and bought a $250 Nextel phone instead.

It is the same recourse we all have in the event that the carrier decides to make a material change to your access to their network. This isn't much consolation to SERO owners however, and while we're slowly being weeded out with new phones and technologies, it's important to realize that you always have a choice. One can't be forced to change to a more expensive plan, and as long as this is true, it's a fair game in their and regulators' eyes (sadly enough).

But realistically speaking, how long can a SERO/TP2 combo be a viable option for today's consumer?