Can't change Instance's Family. Try changing Instance's Host to a Level first.
Namespace: Autodesk.Revit.DB
Assembly: RevitAPI (in RevitAPI.dll) Version: 26.1.0.0 (26.1.0.34)
Syntax
C#
public static FailureDefinitionId CannotChangeInstFam { get; }
BuiltInFailures.FamilyFailures.CannotChangeInstFam Property
Namespace: Autodesk.Revit.DB
Assembly: RevitAPI (in RevitAPI.dll) Version: 26.1.0.0 (26.1.0.34)