The same conditions that supported it, continuing to be true. Canadian citizenship does not change; the licence, the employer and the nature of the work all can. The profession must keep describing what she actually does, the licence must remain current, and the employment must continue with the employer named in the application.
Eligibility is a present-tense test, repeatedly
It is tempting to treat the first successful crossing as the qualifying event and everything afterwards as administration. The classification does not work that way. At every extension, and in substance at every re-entry, the question is whether the person before the officer is a citizen of Canada or Mexico engaged in prearranged professional work in a listed profession, holding whatever that profession's rule requires.
Citizenship is stable. The other elements are not. A licence lapses if continuing competence is not recorded.
An employer restructures and the role becomes a coordination position with a clinical title. An agency reassigns the placement to a different contracting entity. Any of those can leave the current facts outside what was approved, without anybody making a decision to change anything.
The practical discipline is an annual review: read the original letter, compare it with the present job description, check the licence status, and correct any drift while it is still small.