Creating existing events by Exchange Web Services - exchange-server

Question

I'm moving a business from Kerio Connect to MS Exchange 2010 (don't ask why). So I wrote some code that pulls events and tasks from Kerio by CalDAV and recreating them in Exchange by using EWS (Exchange Web Services).
Everything works fine except one thing: the link between the organizer and participants is broken. By that, I mean that a participant can edit the meeting, which he shouldn't be able to do, and if the organizer change the meeting time, participants don't get the update.
I have looked at what Outlook 2011 and iCal does when creating events, and I do the same steps, except for one important thing: the events I recreate in Exchange are events where participants already accepted invitations, and we don't want to send invitations when we start the migration.
This is the request to create the event in EWS:
<S:Envelope xmlns:S="http://schemas.xmlsoap.org/soap/envelope/">
<S:Header>
<ns2:MailboxCulture xmlns:ns2="http://schemas.microsoft.com/exchange/services/2006/types" xmlns:ns3="http://schemas.microsoft.com/exchange/services/2006/messages">en-US</ns2:MailboxCulture>
<ns2:RequestServerVersion xmlns:ns2="http://schemas.microsoft.com/exchange/services/2006/types" xmlns:ns3="http://schemas.microsoft.com/exchange/services/2006/messages" Version="Exchange2010_SP1" />
<ns2:TimeZoneContext xmlns:ns2="http://schemas.microsoft.com/exchange/services/2006/types" xmlns:ns3="http://schemas.microsoft.com/exchange/services/2006/messages">
<ns2:TimeZoneDefinition Id="Eastern Standard Time" />
</ns2:TimeZoneContext>
</S:Header>
<S:Body>
<ns3:CreateItem xmlns:ns2="http://schemas.microsoft.com/exchange/services/2006/types" xmlns:ns3="http://schemas.microsoft.com/exchange/services/2006/messages" SendMeetingInvitations="SendToAllAndSaveCopy">
<ns3:SavedItemFolderId>
<ns2:FolderId Id="AAMkADZkZDFkMjcwLTM5YWItNGRiYy05YzA0LWZhZmM3MjBkNDM1ZgAuAAAAAAAQdcJHRof+QosJDrhZyY00AQCw+XICdccgRJNrvNFaYvpgAAAAevvIAAA=" />
</ns3:SavedItemFolderId>
<ns3:Items>
<ns2:CalendarItem>
<ns2:Subject>Test prob -> probert</ns2:Subject>
<ns2:Body BodyType="Text">
</ns2:Body>
<ns2:Importance>Normal</ns2:Importance>
<ns2:UID>3CB0FF7D-63F0-47A0-80B6-3B3131E7BCF4</ns2:UID>
<ns2:Start>2012-12-21T17:00:00</ns2:Start>
<ns2:End>2012-12-21T18:30:00</ns2:End>
<ns2:IsAllDayEvent>false</ns2:IsAllDayEvent>
<ns2:Location>salle3219_6</ns2:Location>
<ns2:RequiredAttendees>
<ns2:Attendee>
<ns2:Mailbox>
<ns2:Name>Pascal Robert</ns2:Name>
<ns2:EmailAddress>probert#mydomain.com</ns2:EmailAddress>
</ns2:Mailbox>
<ns2:ResponseType>Accept</ns2:ResponseType>
</ns2:Attendee>
</ns2:RequiredAttendees>
<ns2:OptionalAttendees>
<ns2:Attendee>
<ns2:Mailbox>
<ns2:Name>salle3219_6</ns2:Name>
<ns2:EmailAddress>salle3219_6#mydomain.com</ns2:EmailAddress>
</ns2:Mailbox>
<ns2:ResponseType>Accept</ns2:ResponseType>
</ns2:Attendee>
</ns2:OptionalAttendees>
</ns2:CalendarItem>
</ns3:Items>
</ns3:CreateItem>
</S:Body>