Short answer. Not for the failure-to-qualify ground. The Labor Code ties that ground to reasonable standards made known to you at the time you were engaged, not standards introduced or altered later. If new criteria surface midway through your probation, a dismissal based on those later standards sits outside what the article describes.
What the law says
The services of an employee who has been engaged on a probationary basis may be terminated for a just cause or when he fails to qualify as a regular employee in accordance with reasonable standards made known by the employer to the employee at the time of his engagement.
Labor Code, Article 281 — Probationary Employment. Read the full provision →
The standards are fixed to a moment, not a moving target
Article 281 conditions the failure-to-qualify ground on standards made known by the employer to the employee at the time of his engagement. The phrase points to a single moment — when you were hired — not to whatever criteria the employer may settle on as probation goes along. On the article's own wording, the standards a probationary dismissal can be measured against are the ones that existed and were communicated at that starting point.
What a mid-probation change does to that ground
If your employer introduces new or different standards partway through the six months, those later standards were not made known to you at the time of engagement. A termination that rests on failing to meet standards you only learned of midway through probation does not match what Article 281 describes for the failure-to-qualify ground, because the timing condition the article sets is not met by standards disclosed after you already started working.
Two conditions, and what happens if neither was met
The phrase carries two requirements at once, and the timing one is only the second. The standards must be reasonable, and they must have been made known by the employer to the employee at the time of his engagement — an unreasonable standard disclosed on day one fails the test just as surely as a reasonable one sprung in month four. Where nothing was communicated at engagement at all, the failure-to-qualify ground is left with nothing to measure you against, and the employer who wants to end the employment is thrown back on the just-cause ground instead. The last sentence of Article 281 sets the outer limit on all of this: an employee who is allowed to work after a probationary period shall be considered a regular employee. Revising the standards late in the six months therefore buys the employer no extra time; the clock keeps running while the yardstick is being rewritten.
This does not touch the just-cause ground
Article 281's failure-to-qualify ground and its just-cause ground are separate. Nothing about changed standards affects the just-cause ground, which does not depend on standards being disclosed at hiring at all. So while a mid-probation change in standards is not what the article contemplates for judging whether you "qualified," it does not remove the employer's separate ability to act on a genuine just cause if one exists. It is worth separating two things the article treats differently, though. Nothing here stops an employer from giving instructions, setting targets or correcting your work as probation goes on — that is ordinary direction of the work. What the article fixes is the yardstick a failure-to-qualify dismissal may be measured against, not the employer's day-to-day authority.
What is worth keeping
Keep whatever document set out your original standards at hiring, and note when and how any revised standards were communicated to you. If a dismissal is based on criteria you first heard of partway through the six months, that gap between the original standards and the later ones is exactly what Article 281's timing requirement is meant to address, and it is worth writing down the dates while they are still fresh in your memory.