That will return the version of the host application (Reader/Acrobat) and it is readonly. If you are versioning your form at design time (Designer) as well as run time (Acrobat) create a version property for the Info object: this.info.version="v1" (Acrobat context) or event.target.info.version="v1" (xfa context).
The good thing about using the Info object is the property exists both in the pdf architecture as well as the xfa xmpmeta data object model.
You can even create it in Designer under File>Form Properties>Info and create a custom property 'version' with any value you chose.
Kyle