The problem
A platform change is easy to announce and hard to trust. The cluster can be healthy while the answers quietly drift.
The approach
I have taken search through Solr version upgrades and through a move from self-managed Solr to AWS managed OpenSearch, with no downtime on that production cut. Relevancy parity was checked with Jaccard overlap on top results, and with like-for-like displacement where the product returns one record.
The conclusion
Finish the migration when the results agree, not when the dashboard is green. The same habit applies to hybrid retrieval: measure the blend, do not assume the new signal is kinder.