The current code for the drive.mount() function is found at https://github.com/googlecolab/colabtools/blob/fe964e0e046c12394bae732eaaeda478bc5fa350/google/colab/drive.py
It is a wrapper for the drive executable found at /opt/google/drive/drive. I have found that executable accepts a flag authorize_new_user which can be used to force a reauthentication.
Copy and paste the contents of the drive.py file into your notebook. Then modify the call to d.sendline() currently on line 189 to look like this (note the addition of the authorize_new_user flag):
d.sendline(
('cat {fifo} | head -1 | ( {d}/drive '
'--features=max_parallel_push_task_instances:10,'
'max_operation_batch_size:15,opendir_timeout_ms:{timeout_ms},'
'virtual_folders:true '
'--authorize_new_user=True '
'--inet_family=' + inet_family + ' ' + metadata_auth_arg +
'--preferences=trusted_root_certs_file_path:'
'{d}/roots.pem,mount_point_path:{mnt} --console_auth 2>&1 '
'| grep --line-buffered -E "{oauth_prompt}|{problem_and_stopped}"; '
'echo "{drive_exited}"; ) &').format(
d=drive_dir,
timeout_ms=timeout_ms,
mnt=mountpoint,
fifo=fifo,
oauth_prompt=oauth_prompt,
problem_and_stopped=problem_and_stopped,
drive_exited=drive_exited))
Call either the drive module version of flush_and_unmount() or the one you pasted in, and then call your version of mount() to login as a different user!