This is a great question. Let me try to answer it for you.
PTC offers and supports a number of Toolkit libraries - Creo Toolkit (C), Creo Object Toolkit (C++) and Creo Java Object Toolkit. We have no plans to retire any of these Toolkit libraries. Therefore, one will not be replaced by another......so customers and partners can continue to develop their applications with the desired library. Putting Java Object Toolkit aside, any new function in a future release will be delivered in the Creo Object Toolkit Library. It WILL NOT be added to the traditional Creo Toolkit. In addition, in each future release, we are looking to provide equivalent functions in Creo Object Toolkit that are found in Creo Toolkit. Our goal is to have more customers/partners transition to Creo Object Toolkit over time. It is our belief that, Creo Object Toolkit will provide a better environment for you to develop your applications.
So to answer your question, we will not replace Creo Toolkit with Creo object Toolkit in Creo 6.0. You might find you will use both together if you have existing applications that leverage Creo Toolkit, but need new functions from Object Toolkit. Note: you cannot call the same function from both libraries.....you would only be able to call one or the other.
It would also be beneficial to use the Toolkit Utility to help determine those API's that might have been deprecated in your code and replace them with the replaced function.
Hope this helps.