Question: Unlike reptiles or some solid fish we as human race are very sensitive to temperature. A small difference in our body temperature, of only 1 single Celsius degrees causes fever, 4 degrees and we start to lose our mental abilities, more and we are dead. 

While on the other hand, cold blooded animals can't produce their own heat and are too dependent on their environment, plus both parasitic and beneficial bacteria have a harder time surviving inside cold blooded organisms.

So, what changes does human biology need to allow a person's survival and intelligence with cold blood? 

What could force humanity to evolve into cold blood animals?

Answer: 
There is only one **mammal** that can [function reasonably within a very wide temperature range](https://en.wikipedia.org/wiki/Poikilotherm), the naked mole-rat. 

...which means that "cold-bloodedness" stopped being a thing with our part of the family tree about 200 million years ago, when [the mammalian branch was formed](https://en.wikipedia.org/wiki/Evolution_of_mammals).

...which means that **you need to unwind and redo 200 millions years of evolution**.

...which means that the answer to your question "What needs to change?" is: "pretty much everything that concerns our metabolism". Whatever the result is, do not expect them to be humans, in the sense Homo Sapiens, but an entirely new species.
To achieve cold bloodedness in the first place, I would suggest the removal of the hypothalamus, though this comes with its own problems.

<https://www.quora.com/What-does-the-hypothalamus-do-What-happens-if-it-is-removed>

It may be necessary to do this to babies before they are born, as this artical suggests the pancreas develops with the hypothalamus in the womb, which means it may be too late by the time the child is born.

<http://www.ncbi.nlm.nih.gov/pubmed/7019801>

It may be possible to eliminate the symptoms of missing a hypothalamus with a specific form of HRT (hormone replacement therapy) to replace the missing hormones at set intervals or as and when required. You would also require insulin injections in order to properly process sugars.

Now that you have no way to regulate your own body temperature I would suggest moving somewhere warm and keeping wrapped up. Also live near water in case you find you are too hot and need to cool yourself. Also a change in diet would probably be beneficial to aid your self inflicted diabetes.

To make the changes more permanent, a selective breeding program between  survivors of your barbaric experiments  members of your new community to see if you can naturally reduce the effects of the hypothalamus on bodyheat over the generations.