How to replace a subprocess element on a pallet

First, have a look at bpmn-js-example-custom-controls. It shows how to add a new entry to the palette.

Then, have a look at the following topic that shows how to remove entries from the default palette.

If you don’t want to remove the default entry and add a new one, but actually replace the existing one, you can still do it using what you learnt from the linked resources.