We clone our two sub-prod instances (Dev/QA) after an upgrade, and every release, so that when developing in Dev, we know we're building on top of a recent approximation of Prod, and there won't be any unexpected surprised caused by version discrepancies when we promote to QA then Prod. We leave one other instance (sandbox) on the previous version for a couple of weeks in case it helps with troubleshooting any post-upgrade issues.
