I had a similar issue and regex mapping seemed to work for me, try something like this:
{
"$type": "RegexFieldMapConfig",
"WorkItemTypeName": "*",
"sourceField": "System.AreaPath",
"targetField": "System.AreaPath",
"pattern": "SourceProj\\\\(.*)",
"replacement": "TargetProj\\NewParentNode\\$1",
"valueMapping": { "SourceProj" : "TargetProj\\NewParentNode\\$1" }
},
{
"$type": "RegexFieldMapConfig",
"WorkItemTypeName": "*",
"sourceField": "System.IterationPath",
"targetField": "System.IterationPath",
"pattern": "SourceProj\\\\(.*)",
"replacement": "TargetProj\\NewParentNode\\$1",
"valueMapping": { "SourceProj" : "TargetProj\\NewParentNode\\$1" }
},
{
"$type": "RegexFieldMapConfig",
"WorkItemTypeName": "*",
"sourceField": "System.NodeName",
"targetField": "System.NodeName",
"pattern": "SourceProj\\\\(.*)",
"replacement": "TargetProj\\NewParentNode\\$1",
"valueMapping": { "SourceProj" : "TargetProj\\NewParentNode\\$1" }
}