Breaking Changes & Migration Risks in SQL Server 2025

Every new SQL Server release comes with shiny features — but SQL Server 2025 brings more than just enhancements. It's important to know that there are several breaking changes under the hood that could futz your upgrade if you’re not paying attention.

The New Reality

SQL Server 2025 marks Microsoft’s biggest structural shift since 2019. Many legacy subsystems are being removed or rewritten to make room for AI-driven query features, better JSON handling, and tighter security boundaries. That progress comes at a cost, especially for older or hybrid environments.

What’s Breaking or Disappearing

• Hot-Add CPU and Lightweight Pooling. Both are deprecated and no longer supported in 2025.
• Master Data Services (MDS) and Data Quality Services (DQS). Officially retired; time to look toward Azure Purview or Fabric.
• Remote Server connections. Linked Server encryption rules now enforce modern TLS by default, breaking many legacy connections.
• Replication & Log Shipping authentication. May fail under new encryption defaults unless reconfigured with certificates or updated service accounts.
• Extended stored procedures. Custom DLL calls are now fully blocked; migrate to CLR or external services.

Security & Connectivity Changes

Microsoft has quietly raised the security baseline. If your environment still uses older SQL Native Client providers, you’ll see connection errors after upgrade. The new defaults enforce:

• TLS 1.2+ only (no fallback)
• Encrypted channel by default for Linked Servers
• Hardened credentials in system views

That means every legacy Linked Server, SSIS package, or service account using old OLE DB providers needs to be tested and re-registered. Don’t make the assumption that it's just going to reconnect. It won't.

Collation & Compatibility

Collation rules have evolved again. Unicode normalization is now more aggressive, and Latin1_General_100_CI_AI_SC_UTF8 behaves differently in ordering and comparisons. Even small changes in sort order can break ETL checksums or equality joins when migrating from 2017/2019 builds.

Query Engine Behavior

SQL Server 2025 introduces deeper refinements in the Intelligent Query Processing framework and smarter cost model calibration. These changes continue the trend that began in 2019 and 2022 — giving the optimizer more freedom to adapt to runtime feedback and plan variability. While performance generally improves, some workloads may behave differently as a result of:

• Re-estimated row counts during parameter-sensitive execution plans (via new IQP features).
• Reordered joins and aggregations under the updated cardinality model.
• Deprecated or ignored trace flags that previously influenced join behavior or costing.

SQL Server 2025 also expands the Optional Parameter Plan Optimization (OPPO) feature, which lets the engine build multiple parameter-specific plans for a single query. That can dramatically improve parameter-sniffing stability — but it can also cause unexpected plan shifts compared to previous compatibility levels.

To stay safe: capture baselines in Query Store, compare plan regressions under compatibility level 170, and validate with representative workloads before cutting over to production.

Always test your workload under DBCC TRACEON(3604, 8675) or via Query Store captures before committing. You might very well find that previously stable queries are taking new paths now -- possibly not the right ones!

Migration Risk Checklist

• ✅ Validate every Linked Server using new TLS policies.
• ✅ Review replication agents and connection strings.
• ✅ Rebuild SSIS packages that use older OLE DB drivers.
• ✅ Test collation-dependent logic and string joins.
• ✅ Re-benchmark key workloads in compatibility level 170.

Final Thoughts

SQL Server 2025 isn’t a simple “upgrade.” It’s a platform shift toward secure, AI-ready data operations... whether you want them or not.

For DBAs, the best migration plan is simple: know your dependencies, test everything twice, and never assume yesterday’s connection string will work tomorrow.

More to read: Optional Parameter Plan Optimization