ISO 20000:2011 -> ISO 20000:2018 transition

Answer:
First of all, investment made in ISO 20000 (according to 2011 revision of the standard) is not vested effort. Many elements are "recyclable". There are some parts that are simplified but there are also some new requirements.
First of all - new revision follows High Level Structure (HLS), so if you have any other management system in place, it will be very usable in ISO 20000 transition.
Documentation requirements are less than in 2011 revision. But, most of the documents can be reused although not directly required (e.g. Capacity Plan - it's not required to document it but you need to plan it).
Few new requirements (e.g. Context of the organization or Knowledge Management) will need to be addressed but experience with other management systems helps.
Process requirements - many of the m are simplified. That leaves more space for the implementation according to your own organizational setup (or some see it as - lowering bureaucracy).
Note that there are some items that are renamed, like CMDB - focus is on Configuration information.
You will also need to split some processes, like Incident and Service Request Management.

So, yes there are changes but nothing you that invincible.