Making changes to the cdt which is already in use

Suppose i am using a CDT in my process model and i spawn a few 100-200 instances of this process model,Later once these instances are in inflight mode i realise i missed out a field in my CDT which was very important one .Then i make changes to the cdt and publish the new cdt . The later instances which i spawn will point to the newer version of the cdt .But the older instances 100-200 will still keep pointing to the older version of the CDT . In such case what measures can we take to minimise the loss because i cant just cancel all the older instances and request the client to create new instances

Please suggest