How to manage a bookings site after the owner leaves the organization

SubhamKSinha 

Thanks. I missed that one :
Add-RecipientPermission -Identity <bookingmailbox@emailaddress> -Trustee <adminusers@emailaddress> -AccessRights SendAs -Confirm:$false

In summary :

1. Extract the list of all Bookings : get-mailbox -RecipientTypeDetails scheduling
2. Add this permission : Add-MailboxPermission -Identity <bookingmailbox@emailaddress> -User <adminusers@emailaddress> -AccessRights FullAccess -Deny:$false -AutoMapping:$false
3. Add this last permission : Add-RecipientPermission -Identity <bookingmailbox@emailaddress> -Trustee <adminusers@emailaddress> -AccessRights SendAs -Confirm:$false

And I create a little script to find booking without staff on it:

$AllBookingsMailbox = get-mailbox -RecipientTypeDetails scheduling

foreach ($BookingMailbox in $AllBookingsMailbox)
{
$Permissions = Get-MailboxPermission -Identity $BookingMailbox.Identity
$NbPermissions = $Permissions.Count
#If only 2 permissions that is the default ones
If ($NbPermissions -eq 2)
{
Get-MailboxPermission -Identity $BookingMailbox.Identity |Format-List
}
}

SubhamKSinha I tried all that but when I try to apply any changes, they don't apply.  I don't want to delete the site, I want to add someone else as admin.  The only way I've been able to get close to what I want to do, is to make a copy of the site, and then everything works fine.