I have written about default lakehouse of a Fabric notebook before here and here. However, unless you used the notebook API, there was no easy/quick way of removing all/selective lakehouses or updating the default lakehouse of a notebook. But thanks to tip from Yi Lin from Notebooks product team, notebookutils.notebook.updateDefinition has two extra parameters, defaultLakehouse and defaultLakehouseWorkspace which can be used to update the default lakehouse of a notebook. You can also use it to update environment attached to a notebook. Below are some scenarios how it can be used.

Get Notebook Definition:

Use the notebookutils.notebook.getDefinition to get the notebook in a ipynb format as below:

import json

nb = json.loads(
    notebookutils.notebook.getDefinition(
        "<notebook_name>", #nane of the notebook
        workspaceId='<workspace_id_notebook>' #workspaceid of the notebook
    )
) 
#returns a json with ipynb

Get the default lakehouse of a notebook:

import json

nb = json.loads(
    notebookutils.notebook.getDefinition(
        "<notebook_name>", #nane of the notebook
        workspaceId='<workspace_id_notebook>' #workspaceid of the notebook
    )
) ['metadata']

## Returns
"""
{'language_info': {'name': 'python'},
 'kernelspec': {'name': 'synapse_pyspark',
  'language': 'Python',
  'display_name': 'Synapse PySpark'},
 'kernel_info': {'name': 'synapse_pyspark'},
 'widgets': {},
 'microsoft': {'language': 'python',
  'language_group': 'synapse_pyspark',
  'ms_spell_check': {'ms_spell_check_language': 'en'}},
 'nteract': {'version': 'nteract-front-end@1.0.0'},
 'spark_compute': {'compute_id': '/trident/default'},
 'dependencies': {'lakehouse': {'default_lakehouse': 'af9de4fd-e3-772099b80f9d',
   'known_lakehouses': [{'id': 'af9de4fd--772099b80f9d'},
    {'id': 'ab14c0f6-4f284707'},
    {'id': 'c9c60586-dc95ac3c'},
    {'id': '63c748e6-7b34c7f3'},
    {'id': '1d289df0-b9585c'},
    {'id': '12414a55-cd3be'}],
   'default_lakehouse_name': 'workspace1',
   'default_lakehouse_workspace_id': 'd70c1ed700a13145ff'},
  'environment': {}}}
"""

As you can see above, the definition returns a list of all the lakehouses mounted in the notebook, including the default lakehouse.

Remove All Lakehouses

To remove all lakehouses, we can remove all known and default lakehouses as below:

import json

def remove_all_lakehouses(notebook_name, workspace_id):
    try:
        nb = json.loads(notebookutils.notebook.getDefinition(notebook_name, workspaceId=workspace_id))
    except:
        print("Error, check notebook & workspace id")

    if 'dependencies' in nb['metadata'] and 'lakehouse' in nb['metadata']['dependencies']:
        # Remove all lakehouses
        nb['metadata']['dependencies']['lakehouse'] = {}

    # Update the notebook definition without any lakehouses
    notebookutils.notebook.updateDefinition(
        name=notebook_name,
        content=json.dumps(nb),
        workspaceId=workspace_id
    )

    print(f"All lakehouses have been removed from notebook '{notebook_name}'.")

# e.g.
remove_all_lakehouses("Notebook 1", "<workspaceid_of_notebook>")

Add A New Default Lakehouse

To add a new lakehouse and make it a default lakehouse without removing any existing alkehouses:

(notebookutils
    .notebook
    .updateDefinition(
        name = "<notebook anme>", 
        workspaceId="<worskpace id of the notebook>"
        defaultLakehouse="<lakehouse name of the new lakehouse>", 
        defaultLakehouseWorkspace="<workspace id of new lakehouse to be added>", 
        )
)
# content parameter is optional which keeps the existing content

Remove All Lakehouses And Add New Lakehouse

To remove all existing lakehouses attached to the notebook and make a new lakehouse the default lakehouse:

# first rmeove the lakehouses
remove_all_lakehouses("Notebook_name", "workspaceid_of_notebook")

#add new lakehouse
(notebookutils
    .notebook
    .updateDefinition(
        name = "Notebook_name", 
        workspaceId="workspaceid_of_notebook"
        defaultLakehouse="<lakehouse name of the new lakehouse>", 
        defaultLakehouseWorkspace="<workspace id of new lakehouse to be added>", 
        )
)

Needless to say, when you make these changes, be sure to reference to the right paths in your code. You can use the .lakehouse for it (see here).