The 30 second answer
After the changes, I rerun exactly the same load test, with the same data, request rate and environment, and compare p50, p95, p99, error rate and throughput with the baseline. Only that comparison proves the improvement. If I changed the test, the numbers would not be comparable, so the test setup is treated as part of the experiment.
Use the same script, the same data and the same environment, then compare p50, p95, p99 and throughput. k6 thresholds can fail a CI run on regression.