Mailmerge Field not always saved the same way in Word .docx

Question

I have created a Word document with Word 2003 and inserted some MergeField via the GUI. I have saved it as a .docx by using Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint 2007 File Formats. Some Mergefields are stored as a SimpleField, while others are stored as a FieldCode (with start-FieldChar and end-FieldChar). Some Googling brought me to this blog. As you can see the guy is facing the same problem; but hasn't found a solution yet.

I'm using the following code sample on Codeplex [Fill Mergefields] to replace the MergeFields with the actual values from different datasources.

Any help is welcome.