Inject custom type conversion to .NET library classes

You can change the registered TypeConverter for something using TypeDescriptor.AddAttributes; this isn’t quite the same as Convert.ChangeType, but it may suffice:

using System;
using System.ComponentModel;
static class Program
{
    static void Main()
    {
        TypeDescriptor.AddAttributes(typeof(Guid), new TypeConverterAttribute(
            typeof(MyGuidConverter)));

        Guid guid = Guid.NewGuid();
        TypeConverter conv = TypeDescriptor.GetConverter(guid);
        byte[] data = (byte[])conv.ConvertTo(guid, typeof(byte[]));
        Guid newGuid = (Guid)conv.ConvertFrom(data);
    }
}

class MyGuidConverter : GuidConverter
{
    public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
    {
        return sourceType == typeof(byte[]) || base.CanConvertFrom(context, sourceType);
    }
    public override bool CanConvertTo(ITypeDescriptorContext context, Type destinationType)
    {
        return destinationType == typeof(byte[]) || base.CanConvertTo(context, destinationType);
    }
    public override object ConvertFrom(ITypeDescriptorContext context, System.Globalization.CultureInfo culture, object value)
    {
        if (value != null && value is byte[])
        {
            return new Guid((byte[])value);
        }
        return base.ConvertFrom(context, culture, value);
    }
    public override object ConvertTo(ITypeDescriptorContext context, System.Globalization.CultureInfo culture, object value, Type destinationType)
    {
        if (destinationType == typeof(byte[]))
        {
            return ((Guid)value).ToByteArray();
        }
        return base.ConvertTo(context, culture, value, destinationType);
    }
}