As the title suggest, see below a snippet for a conversation from the trace for "FindConversation" against Office365.
As you can see, the ChangeKey is "CQAAAA==" for all the items (across conversations), and interesting enough, it is the same as in the documentation for FindConversation
https://msdn.microsoft.com/en-us/library/office/ff406144(v=exchg.150).aspx
Is this a known issue?
<Conversation xmlns="http://schemas.microsoft.com/exchange/services/2006/types"><ConversationId Id="AAQkADMwZjkyMjExLTMzMGUtNGU5MS04OWIzLTAyY2ExYjM2ZjFiMgAQAB2NeP3GTQlDp5B7AWesrrw=" /><ConversationTopic>Kid's turning the dog green again</ConversationTopic><UniqueRecipients><String>John Smith</String></UniqueRecipients><GlobalUniqueRecipients><String>John Smith</String></GlobalUniqueRecipients><UniqueSenders><String>John Smith</String></UniqueSenders><GlobalUniqueSenders><String>John Smith</String></GlobalUniqueSenders><LastDeliveryTime>2014-11-26T16:21:37Z</LastDeliveryTime><GlobalLastDeliveryTime>2014-11-27T14:09:09Z</GlobalLastDeliveryTime><HasAttachments>false</HasAttachments><GlobalHasAttachments>false</GlobalHasAttachments><MessageCount>1</MessageCount><GlobalMessageCount>4</GlobalMessageCount><UnreadCount>0</UnreadCount><GlobalUnreadCount>0</GlobalUnreadCount><Size>16099</Size><GlobalSize>28992</GlobalSize><ItemClasses><ItemClass>IPM.Note</ItemClass></ItemClasses><GlobalItemClasses><ItemClass>IPM.Note</ItemClass></GlobalItemClasses><Importance>Normal</Importance><GlobalImportance>Normal</GlobalImportance><ItemIds><ItemId Id="AAMkADMwZjkyMjExLTMzMGUtNGU5MS04OWIzLTAyY2ExYjM2ZjFiMgBGAAAAAAC/m9YT3n+7RbfoU4MvIUlwBwDO121Po4OqQ7Wo59kdWfWgAAAAAAEMAADO121Po4OqQ7Wo59kdWfWgAAASN40cAAA=" ChangeKey="CQAAAA==" /></ItemIds><GlobalItemIds><ItemId Id="AAMkADMwZjkyMjExLTMzMGUtNGU5MS04OWIzLTAyY2ExYjM2ZjFiMgBGAAAAAAC/m9YT3n+7RbfoU4MvIUlwBwDO121Po4OqQ7Wo59kdWfWgAAAAAAEJAADO121Po4OqQ7Wo59kdWfWgAAANAp25AAA=" ChangeKey="CQAAAA==" /><ItemId Id="AAMkADMwZjkyMjExLTMzMGUtNGU5MS04OWIzLTAyY2ExYjM2ZjFiMgBGAAAAAAC/m9YT3n+7RbfoU4MvIUlwBwDO121Po4OqQ7Wo59kdWfWgAAAAAAEJAADO121Po4OqQ7Wo59kdWfWgAAANAp24AAA=" ChangeKey="CQAAAA==" /><ItemId Id="AAMkADMwZjkyMjExLTMzMGUtNGU5MS04OWIzLTAyY2ExYjM2ZjFiMgBGAAAAAAC/m9YT3n+7RbfoU4MvIUlwBwDO121Po4OqQ7Wo59kdWfWgAAAAAAEJAADO121Po4OqQ7Wo59kdWfWgAAANAp23AAA=" ChangeKey="CQAAAA==" /><ItemId Id="AAMkADMwZjkyMjExLTMzMGUtNGU5MS04OWIzLTAyY2ExYjM2ZjFiMgBGAAAAAAC/m9YT3n+7RbfoU4MvIUlwBwDO121Po4OqQ7Wo59kdWfWgAAAAAAEMAADO121Po4OqQ7Wo59kdWfWgAAASN40cAAA=" ChangeKey="CQAAAA==" /></GlobalItemIds><LastModifiedTime>2014-12-05T19:12:48Z</LastModifiedTime><InstanceKey>AQAAAAAAAQABAAAADQKSGAAAAAA=</InstanceKey><HasIrm>false</HasIrm><GlobalHasIrm>false</GlobalHasIrm></Conversation>