When the Buddha changes his mind to teach I remember reading a sutta (possibly in Majjhima or Samyutta) where a passage has the Buddha talking about his teaching attitude towards certain people when talking to them. Then, he says something to the effect of "... but when a person is a fool, the Tathagata changes his mind".

What sutta is this?

I'm positive it's not AN 4.111, where there's an analogy with horse trainning:


  “[...] But if a tamable person doesn’t submit either to a mild training or to a harsh training or to a mild &amp; harsh training, then the Tathagata doesn’t regard him as being worth speaking to or admonishing.
It is the Asibandhakaputta Sutta- SN 4.8.6
It was Asibandhakaputta , the disciple of Nighanta-Natha-Putta, who raised the question for which Buddha gave the
parable of the three fields, the good , the middling, and the poor.   In this parable Buddhist laity rank as the moderate field, between the Bhikkus (excellent) and the men of other creeds (poor).

Within this Sutta look for the Khettûpama Sutta - SN 4.8.7. where Buddha took this parable of the three fields: the greatly fertile field, the fertile field, and, the unfertile field.   This Sutta explains as to why the Buddha does not teach all people with the same level of effort. The greatly fertile field is prioritized over the fertile field and the fertile field is prioritized over the unfertile field.  The greatly fertile field as monks and nuns; the fertile field as laymen and laywomen; and, the unfertile field being adherents of other doctrine and methods. It also includes the parable of the three pots; the parable of righting the upside down thing; the parable of the uncovering; the parable of the right road; the parable of the lighted lamp.


  “Venerable Sir, doesn’t the Blessed One dwell compassionate towards all living beings?”  
  
  “Yes, headman, the Tathagata dwells compassionate towads all living beings.” 
  
  “Then why is it, venerable Sir, that the Blessed One teaches the Dhamma thoroughly to some, yet not so thoroughly to others?  
  
  “Well then headman, I will question you about this. Answer as you see fit. What do you think headman? Suppose a farmer here had three fields: one excellent, one of middling quality, and one inferior – rough, salty, with bad ground. What do you think, headman? If that farmer wishes to sow seed, where would he sow it first… in the excellent field, in the field of middling quality, or in the field that was inferior – rough, salty, with bad ground?


You could read further on this in the Connected Discourses of the Buddha pages 1338, 1339, 1340.