I don't use the LDAP ActiveSync adapter, but if it's the same as others we use, you should be able to edit the IAPI object for the resource.
In Debug, list objects of type Configuration and look for IAPI_YourResourceName. Edit this object and look for a MapEntry which stores the change number and set it to the value you need.
Jason
Found it!
I forgot to mention that we are running IDM 7.0. I spoke with a support person from SUN, and they informed me that the IAPI objects are no longer around in IDM 7.0. Instead the data is stored in XmlData object types. There names start with SYNC_<resourceName>.
Sure enough the number was there.
Thanks again for you feedback.