How to convert between Outlook PST file formats (ANSI and Unicode)?

An archive PST from Outlook 2002 refuses to grow past 2GB and started throwing errors. I gather it is the old ANSI format.

How do I check the format and convert it to the modern one without losing folder structure?

You are right about the cause. ANSI PSTs cap at 2GB and corrupt themselves when they touch the limit. Unicode PSTs, the format since Outlook 2003, go to 50GB by default.

Check the format first: File, Account Settings, Data Files tab, select the file, Settings. If the dialog says Outlook Data File and shows a Format line reading Outlook Data File instead of Outlook 97-2002 Data File, it is already Unicode.

There is no in place convert button. The conversion is a copy into a fresh file:

1

File, New Items, More Items, Outlook Data File. Save it as a new PST, this is created as Unicode automatically.

2

Open both files in the folder pane. Drag each top level folder from the old PST into the new one. Folder structure and items copy intact.

3

Compare item counts: right click each folder, Properties, Show total number of items on both sides.

4

Close and detach the old ANSI file via Account Settings once counts match.

If the ANSI file already crossed 2GB and will not open at all, drag and drop is off the table. A repair tool that reads damaged ANSI files, SysTools Outlook Recovery for instance, can extract the contents straight into a Unicode PST.

Mine was still readable at 1.98GB so the drag across worked. Counts match on all 22 folders. Cutting it close, glad I caught it.