Changing the PDF version with iTextSharp is fairly easy, all you have to do is create a stamper and pass an extra parameter with the PDF version you want. Below is the code snippet to accomplish this in C# and VB.NET.

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C# Code

PdfReader reader = new PdfReader(fileBytes);
MemoryStream ms = new MemoryStream();
PdfStamper stamper = new PdfStamper(reader, ms, "4"); //The number 4 changes the PDF version to 1.4 (or Acrobat 5)
stamper.Close();
fileBytes = ms.ToArray();
ms.Flush();
reader.Close();

VB.Net Code

Dim reader As PdfReader = New PdfReader(fileBytes)
Dim ms As MemoryStream = New MemoryStream()
Dim stamper As PdfStamper = New PdfStamper(reader, ms, "4") 'The number 4 changes the PDF version to 1.4 (or Acrobat 5)
stamper.Close()
fileBytes = ms.ToArray()
ms.Flush()
reader.Close()

Here is a nice table showing the different versions

Acrobat Product Version	PDF Version	New Features Affecting Backwards Compatibility
Acrobat 8	PDF 1.7	Reader Enabled Forms PDF Packages PDF 1.7 XML Forms
Acrobat 7	PDF 1.6	Reader Enabled Review Security Envelopes XML Forms Embedded multimedia including 3D content Bookmark Compression
Acrobat 6	PDF 1.5	Linked multimedia
Acrobat 5	PDF 1.4	OCR Text Layer

Table came from this Adobe blog.

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