Websphere Liberty is in many ways replacing Websphere Application Server.
It is lightweight per default, but can be configured to support pretty much everything WAS can do. Many of the libraries are the same. Websphere Liberty similarly runs on Z/os, if that is a desire.
However the cost of Liberty is lower on server resources and licenses, and is easier to containerize.
We are currently running Entirex 10.7 on Liberty and have been very happy with it so far. As far as I can tell no development effort is required, but some testing may be needed.