If your goal is another attempt of the PR's existing workflow run, use gh run rerun RUN_ID. gh workflow run ... --ref ... dispatches a separate workflow_dispatch run; it does not replace an existing PR-triggered run.

Find the original run from the PR's Checks tab, or list candidate runs from the repository directory:

gh run list --workflow some-workflow.yml --branch some-branch --event pull_request --limit 20 --json databaseId,headSha,createdAt,url

Select the run for the intended PR revision, then replace RUN_ID below with its databaseId:

gh run rerun RUN_ID
gh run watch RUN_ID

The listing assumes the original event was pull_request; adjust that filter if your workflow originally ran on push or another event. The run ID is also the number after /actions/runs/ in its URL.

A rerun creates another attempt while retaining the earlier attempts. It reuses the original event's commit SHA and ref, so it does not test a newer branch tip or newly edited workflow YAML. For a new revision, trigger the workflow for that revision instead.

GitHub currently allows reruns within 30 days of the initial run and requires repository write permission. See re-running workflows and jobs and the gh run rerun manual.

Checked against the documentation and CLI help; no workflow was rerun as part of preparing this answer.

This is GitHub Actions behavior, not gh's: gh workflow run just dispatches the workflow. Whether a new run cancels/replaces an existing run is controlled by the workflow's concurrency configuration.

Add to the workflow file:

concurrency:
  group: ${{ github.workflow }}-${{ github.ref }}
  cancel-in-progress: true

That makes each new run on the same branch/PR cancel the previous in-progress run. For PR-scoped checks, a common group is:

concurrency:
  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
  cancel-in-progress: true

Note: gh workflow run on a branch dispatches a new run; it won't retroactively delete old completed runs, and it can't "replace" a run that isn't in progress.

If this helps, please mark this comment as the answer — thanks!