Can I just buy 1 month just to setup a new phone?

You can cancel an iCloud storage upgrade at any time by downgrading back to the free 5GB plan. See: Downgrade your iCloud storage - Apple Support

After you have used the backup of the old phone to set up the new phone and verified that it is OK, you may want to use the following instructions to delete the old backup from iCloud to free up some storage and possibly make the 5GB plan workable for you. See: Manage your iCloud storage - Apple Support