Obligatory "I am not a lawyer" disclaimer.
Here is a link to the actual, no-kidding, congressionally-passed black letter law govorning the SMART program:
10 USC 2192a.
The bad news (as I interpret it):
For the purposes of this subsection, the period of obligated service for a recipient of financial assistance under this section shall be the period determined by the Secretary of Defense as being appropriate to obtain adequate service in exchange for such financial assistance. The period of service required of a recipient may not be less than the total period of pursuit of a degree that is covered by such financial assistance. The period of obligated service is in addition to any other period for which the recipient is obligated to serve in the civil service of the United States.
Ie, the 1:1 ratio is not actually set by the law. It's at the discretion of the SecDef. I assume that the document we signed represents his determination of an appropriate service period and follows the usual rules of contracts in that it can't be unilaterally changed by one party. But take it as you will.
The good news (as I interpret it):
The Secretary of Defense—(1) may, without regard to any provision of title 5 governing appointment of employees to competitive service positions within the Department of Defense, appoint to a position in the Department of Defense in the excepted service an individual who has successfully completed an academic program for which a scholarship or fellowship under this section was awarded and who, under the terms of the agreement for such scholarship or fellowship, at the time of such appointment, owes a service commitment to the Department; and (2) may, upon satisfactory completion of 2 years of substantially continuous service by an incumbent who was appointed to an excepted service position under the authority of paragraph (1), convert the appointment of such individual, without competition, to a career or career conditional appointment.
Ie, all the usual rules of hiring freezes, billet limitations, and so on can be overriden by the SMART program. This is provided, of course, that the SecDef does not object. But it's my understanding that "The Secretary of Defense may..." in practice means "This is the way it will happen unless he specifically vetoes it".
I'm very interested to hear from people who're more familiar with the law and the inner workings of the program, but I'm hopeful the text of the law will give us some insight.