Class Layout Update on Unity3D

New.cs

using UnityEngine;
using System.Collections;
#if UNITY_EDITOR
using UnityEditor;
#endif

public class Test : MonoBehaviour, ISerializationCallbackReceiver {

	[SerializeField, HideInInspector] int m_SerializedVersion;
	[SerializeField] string m_NewString;
#if UNITY_EDITOR
	string m_Buffer;
#endif
	
	void ISerializationCallbackReceiver.OnBeforeSerialize () {
	}
	
	void ISerializationCallbackReceiver.OnAfterDeserialize () {
#if UNITY_EDITOR
		if (m_SerializedVersion < 1) {
			m_NewString = m_Buffer;
		}

		m_SerializedVersion = 1;
#endif
	}
}

Old.cs

using UnityEngine;
using System.Collections;
#if UNITY_EDITOR
using UnityEditor;
#endif

public class Test : MonoBehaviour, ISerializationCallbackReceiver {

	[SerializeField, HideInInspector] int m_SerializedVersion;
	[SerializeField] string m_OldString;
#if UNITY_EDITOR
	string m_Buffer;
#endif
	
	void ISerializationCallbackReceiver.OnBeforeSerialize () {
#if UNITY_EDITOR
		m_Buffer = m_OldString;
#endif
	}
	
	void ISerializationCallbackReceiver.OnAfterDeserialize () {
#if UNITY_EDITOR
		if (m_SerializedVersion > 0) {
			m_OldString = m_Buffer;
		}


		m_SerializedVersion = 0;
#endif
	}
}

1. Create Test.cs in your project
2. Put the contents of Old.cs in your newly created Test.cs
3. Create a new GameObject, add a Test component to it and put some text on Old String
4. Put the contents of New.cs in Test.cs
5. You will see your New String using the data that was before on Old String
6. You can also go back to Old.cs, and will get the data that you got in New String there

Note that m_Buffer is actually serialized and not saved to disk (i.e. data will survive an assembly reload). This won't work if you mark that field with NonSerializable.