Windows 11 25H2 Is Coming — Prepare Your Legacy Files Now

Windows 11 24H2 broke legacy Office files for thousands of users. Macros stopped running, .xls files refused to open, Access databases threw cryptic errors. The common thread: people didn't know their files were vulnerable until the update had already landed.

Windows 11 25H2 is expected in the second half of this year. Based on every pattern Microsoft has established over the past four years of feature updates, 25H2 will continue tightening restrictions on legacy file formats, VBA macros, and deprecated runtime components. The question isn't whether it will affect legacy files — it's how many of yours are in the blast radius.

This is the prep checklist. Do it now, while your files still work.

Why 25H2 Is Likely to Break More Files

Microsoft hasn't published the full 25H2 compatibility changelog yet. But the trajectory is clear. Each major feature update since Windows 11 22H2 has followed the same pattern:

• 22H2 — Extended Mark of the Web to ZIP-extracted files, blocking macros in files from external sources
• 23H2 — Tightened COM object registration, deprecated several ActiveX control paths
• 24H2 — Removed Jet 4.0 OLE DB provider, expanded MOTW to OneDrive-synced files, hard-blocked deprecated API calls that were previously just warnings

The direction is consistent: legacy formats and legacy runtime components get more restricted with every update. What survived 24H2 may not survive 25H2. And even if it does, it will survive 26H2 with even lower odds.

Step 1: Inventory Your Legacy Files

You can't protect what you don't know about. The first step is finding every legacy Office file on your machines, shared drives, and cloud-synced folders.

The file extensions that matter:

Extension	Format	Risk Level
.xls	Excel 97-2003 (BIFF8)	High
.mdb	Access 97-2003 (Jet 4.0)	Critical
.xlsm with old VBA	Macro-enabled Excel	Medium
.xla	Old Excel add-in	Medium
.xlsx	Modern Excel (no macros)	Low
.accdb	Modern Access (ACE)	Low

To find legacy files across a directory tree, use PowerShell:

Get-ChildItem -Path "C:\YourFiles" -Include "*.xls","*.mdb","*.xla" -Recurse |
    Select-Object FullName, Length, LastWriteTime |
    Export-Csv -Path "legacy-file-inventory.csv" -NoTypeInformation

Run this on every shared drive and local machine. Don't forget OneDrive and SharePoint-synced folders — those are where 24H2 caused the most unexpected breakage because of MOTW flag changes.

Step 2: Scan for Compatibility Issues

Finding the files is step one. Understanding what's inside them is step two. A .xls file with no macros and no ActiveX controls is a different animal from a .xls file with 40 VBA modules, Win32 API declarations, and embedded command buttons.

What you need to know about each file:

• Does it contain VBA macros? If yes, do any modules use Declare statements for Windows API calls?
• Are the Declare statements PtrSafe-compatible? If not, they will fail on 64-bit Office — and 25H2 may enforce this even more strictly.
• Does it use ActiveX controls? Which ones? Are they still registered on your system?
• Does it have external data connections? To Access databases? To ODBC sources using Jet?
• Is the file from a network or cloud source? MOTW policies may block it silently after the update.

Checking all of this manually across dozens or hundreds of files isn't practical. A compatibility scanner does this in minutes.

Step 3: Back Up Everything

Before you migrate anything, create a full backup of every legacy file in its current state. This is your safety net if a migration introduces unexpected changes.

• Store backups in a separate location — not a subfolder on the same drive. An external drive, a different network share, or a dedicated backup folder on a different machine.
• Date-stamp the backup — name it something like pre-25h2-migration-2026-04 so you know exactly when and why it was created.
• Export VBA code separately — open the VBA editor (Alt+F11), right-click each module, and export as .bas files. If a file becomes unreadable later, you can recover the code from these exports.
• For Access databases, also export tables to CSV — this gives you a format-independent copy of the data as a last-resort recovery option.

Step 4: Know Your VBA Dependencies

VBA is where most of the breakage happens during Windows updates. The code itself doesn't change — but the runtime environment it depends on does. Here's what to audit:

Win32 API Declarations

Any Declare Function or Declare Sub statement that calls a Windows DLL directly. These need the PtrSafe keyword for 64-bit compatibility, and pointer-sized parameters need LongPtr instead of Long. If your files still have 32-bit-only declarations, they're already broken on 64-bit Office — and 25H2 is unlikely to make this situation more forgiving.

ActiveX Control References

Check the References dialog in the VBA editor (Tools → References). Any reference marked "MISSING" is a control or library that's already gone from your system. Each Windows update removes or relocates more of these. Resolve them now while you can still find replacements.

External Library Calls

VBA code that calls external DLLs, references COM objects, or uses late binding to create objects (CreateObject("...")) depends on those objects being registered on the machine. Windows updates can change registration paths or remove objects entirely.

Data Connections

If your Excel files pull data from Access databases using Jet 4.0 connection strings (Provider=Microsoft.Jet.OLEDB.4.0), those connections are already broken on 64-bit Windows 11 as of 24H2. Update them to ACE (Provider=Microsoft.ACE.OLEDB.12.0) before 25H2 removes any remaining workarounds.

Step 5: Migrate Before the Update Arrives

The single most effective thing you can do is convert your legacy files to modern formats before 25H2 is installed. Not after. Before.

The migration targets:

• .xls with macros → .xlsm (macro-enabled modern format)
• .xls without macros → .xlsx (modern format)
• .mdb → .accdb (modern Access format, no Jet dependency)
• .xla → .xlam (modern add-in format)

Migrating before the update means you can test on your current stable system. You can open the original and the converted version side by side, run macros, verify formulas, and confirm everything works. Migrating after the update means you're trying to read and convert files on an OS that may no longer fully support the old format.

Step 6: Test on a Pilot Machine

If you have access to a Windows Insider build or a test machine running the 25H2 preview, use it. Copy your migrated files over and verify they open, macros run, data connections work, and ActiveX controls render. If something breaks in testing, you have time to fix it. If something breaks on your production machine the morning after an automatic update, you're in triage mode.

For IT teams managing a fleet, the pilot testing process should include:

1. One machine per department upgraded to the preview build
2. Each department's critical files tested by the primary user
3. Issues documented and resolved before the fleet-wide rollout
4. A rollback plan if critical issues are found that can't be resolved in time

The Bottom Line

Windows 11 25H2 hasn't shipped yet. That's the advantage. You have time to inventory, scan, back up, and migrate your legacy files while they still work on your current system. The organizations that got burned by 24H2 are the ones that didn't take these steps — and found out about their vulnerable files when the update broke them.

The migration effort is real. It takes time and attention. But it's dramatically easier to do proactively, on your own schedule, than reactively, with broken files and a line of users waiting for answers.

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