Earlier quoted context omitted.
> The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. Source: the MIT license. https://choosealicense.com/licenses/mit/
There's no writing in that license which I can't change the copyright after forking the code. There's a copyright line, check. There's the permission notice, check. The rest is just goodwill and ethics, which is not a very valuable currency in software in these days.
Once you change the copyright line, you no longer include "the above copyright notice". At that point you're violating the license.
You are also not allowed to change the copyright notice or license text in any way (you may however add to the license, which is a loophole other licenses such as GPL fix.)
Substantial is subject to (legal) debate as the Oracle vs. MS case has shown. Whole functions or large parts of files however should always be considered substantial, as the software would otherwise not work.
I'm seriously flabbergasted at how bad reading comprehension seems to be among coders.