We have an old approval workflow where some transactions are still in pending approval status. We need to deploy the new approval workflow. How can I trigger those existing transactions? Do I just need to cancel the existing workflow on each transaction and then retrigger it using the new workflow (after setting the old WF to not initiating and inactive)? If so, is there a bulk way to do it especially the cancellation?

You can use Mass Updates to trigger and cancel workflows https://docs.oracle.com/en/cloud/saas/netsuite/ns-online-help/section_N2796043.html