NSF graduate research fellowship eligibility, I've read from past questions and answers on this site, namely an answer by JeffE, that anyone with more than 12 months of full-time graduate study, MS or PhD, is considered too late of an applicant, unless there is a significant change of field.

What if my undergrad degree was not in mathematics, but I have an MS degree in math, and am looking to apply to PhD programs in math?  Would my MS degree be considered something equivalent to an undergrad / first degree in math, and thus make my application eligible for a review by the panelists?
If you have had 12 months of graduate study, then no, you are not eligible, regardless of your undergraduate degree.

Note: there are some exceptions to this (I think they have some language in there for exceptions), but I wouldn't bet on it.