Loading different new mesh in run time

To achieve what you want you need to create your own runtime pak download and mounting system and then swap the assets at runtime using those either BP or C++ examples.
Runtime pak fles mounting seems to be a bit difficult and some discussion threads might help:

https://answers.unrealengine.com/que…runtime-1.html
https://answers.unrealengine.com/que…unted-pak.html
https://answers.unrealengine.com/que…t-runtime.html