Unflatten from XML not working

I want to alter the value inside a variant.  I flatten to XML, manipulate the XML string, then Unflatten from XML.

In the attached code, the starting variant contains an enum whose value is 'OPEN AIR VALVE 1'.  I want to change that to 'OPEN AIR VALVE 3', but it doesn't work, the value stays as 'OPEN AIR VALVE 1'.  What's wrong?