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: 25.3.0.0 (25.3.0.0)
Syntax
C#
public static FailureDefinitionId CannotChangeInstFam { get; }
BuiltInFailures.FamilyFailures.CannotChangeInstFam Property
Namespace: Autodesk.Revit.DB
Assembly: RevitAPI (in RevitAPI.dll) Version: 25.3.0.0 (25.3.0.0)