We have 10000 completed instances of process model version 80.0, which has custo

You could look into using the process upgrade API to update the PV. But have you considered moving to Records, rather than relying on 10,000+ processes for your data?

I guess, process upgrade will work for running process. Here, my required scenario is CDT upgrade on completed processes. Do you have any suggestions how to upgrade the previous CDT to latest version CDT (either API or plug-in) of completed processes?

I can't make any recommendations on how to use the process upgrade API, as I haven't had cause to use it. I don't know what your use-case is, but it seems to me like you could benefit from switching to an entity-backed record design. If you already have a CDT that is linked to a data entity, you can easily make a record from the data entity. Tempo reports can be created off this with queryrecord (or queryentity, if you don't want to create a record, listview, and dashboard). This would allow you to no longer need to deal with the memory requirements and hassle of maintaining 10,000 process instances, which it sounds like you are using for reporting and possible process instance dashboards.