You can use "ProcessInstanceMigrationService"  to migrate process instances from one deployment to another.

The ProcessInstanceMigrationService service enables you to change the process definition for the process engine.

Recommendation is that you should let active process instance finish and than start new process instances in the new deployment.

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public interface ProcessInstanceMigrationService {

/**  Migrates given process instance that belongs to source deployment, into target process id that belongs to target deployment. **/

List migrate(String sourceDeploymentId, List processInstanceIds, String targetDeploymentId, String targetProcessId);

List migrate(String sourceDeploymentId, List processInstanceIds, String targetDeploymentId, String targetProcessId, Map nodeMapping);

}

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You can migrate a single process instance, or multiple process instances at once. If you migrate multiple process instances, each instance will be migrated in a separate transaction to ensure that the migrations do not affect each other. Once migration is done, the migrate method returns MigrationReport.