Personally, I'd have the INCLUDETEXT fields in the document already, and use a DOCPROPERTY field to return a value from a custom document property to determine the output. If the source documents have two bookmarked ranges - one containing the TRUE content and the other containing the FALSE content - all that is needed is to toggle the bookmark reference. For example:
{INCLUDETEXT "C:\\Users\\Genius\\Desktop\\Text1.docx" "{DOCPROPERTY BkMk}"}

Alternatively, if you want two separate documents, use the DOCPROPERTY field to toggle the 1/2 in the document names. For example:
{INCLUDETEXT "C:\\Users\\Genius\\Desktop\\Text{DOCPROPERTY DocID}.docx"}

Either way, you don't need to rely on updating bookmarked ranges in your form document.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]