I looked it up - seemingly a two thirds majority of both houses is required to change the constitution of the USA.
So it's as easy to change the constitution as it is to successfully impeach the President.
You could actually argue it's easier to change the constitution than to impeach president. Only two presidents have been impeached - Clinton and A Johnson - neither were convicted. Whereas, the constitution has been amended frequently.
If impeachment looks likely to result in conviction a president will jump before pushed - Nixon jumped before impeachment from memory.
But realistically what are the chances of getting the constitution amended to allow crimes against humanity?
It's not even reasonable to assume Trump would get all republican representatives to support him on something like that - let alone the democrats.