Where (in an official INM location), pray tell, does it say someone entering MX "has to declare their intentions"? It's not in any document or website I've looked at. ... Others here have cited the third bullet on the reverse side of the FMM form as the source of this misinformation. It reads:

For a foreigner holding a Mexican visa enabling him to obtain an FM3 or FM2, this FMM vouches for his legal stay for 30 days as of entry. IT IS VERY IMPORTANT that you exchange this FMM for an FM2 or FM3 immigration form within the time period indicated at the Regional Delegation of the INM closest to your residence. Otherwise you will be in an irregular situation and will be subject to sanctions provided for in the General Population Law. (note: the INM video explaining how to complete an FMM says the same thing, but refers to and shows a "365 day visa" - which is NOT an FM#).

If any of that means you have to declare your intentions or that one needs to exit and reenter if one changes their mind, you must have learned a different version of English than me. ....