Workflow template changed-Reset all the older workitems

I have changed the workflow template and even after moving it into production all the pending older workitems are still executing the older workflow template

I have around thousands of workitems(Descion task and its relavant workflow) which are triggering older workflow instance.

My duty is to complete all the items and restart Workflow for all these items at a time

Manually completing and restarting via SWUE for each workitem is taking a lot of time.

Could please suggest a way out

One more issue:On manually completing the item the next steps are being executed

But i need to logically delete at that particular step of the workflow and retrigger them.

Thanks in advance