New Post: Using NPOI.SS.UserModel to open both XLS and XLSX for reading

Nevermind.  I just figured it out.  Instead of using POIFSFileSystem, just supply the FileStream directly to WorkbookFactory.Create(...) and it seems to work just fine.

Dim xlWB As IWorkbook = WorkbookFactory.Create(New FileStream(Me.solFilePath, FileMode.Open, FileAccess.Read))

 Sorry for the false alarm.