I think I understand better. I had skimmed over this somehow in the NM instructions:
MARCH 1, 2023
A Finalist who has reported a sponsor college as first choice before March 1 will be included in the first group referred to that institution for scholarship consideration. Only the sponsoring college you have listed as your first choice will be notified.MARCH 1 THROUGH MAY 31, 2023
Periodically NMSC will notify sponsors of additional Finalists who have reported (by May 31) the college or university as their first choice. Only the sponsoring college you list as your first choice will be notified.
So, if March 1 senior year is the earliest date that NMSC will communicate the first choice college to the college, then it doesn’t matter what the student designates until March 1 (I had been thinking of demonstrated interest in fall applications, though this first choice designation seems irrelevant for that because the college wouldn’t know).
(There’s an interesting older thread discussing the competing early deadlines for BU and USC)