All of the more basic DTMF-driven IVR services can be re-created with VoiceXML and the open service architecure that it espouses. The caller isn’t going to notice much of a difference unless you take the migration as an opportunity to rework the user interface.
However, moving from a proprietory IVR to a more flexible modern architecure does bring a lot of technical advantages. The advantage that VoiceXML offers is the speed with which changes can be made and new functionality added. You are no longer dependent on your IVR supplier to make any modifications you require and you can re-use existing IT infrastructure and interfaces. These engineering changes have an effect in terms of the product, as any modifications are now easier and quicker to do, resulting in a faster time2market for better customer-oriented services.
Actually, by randomising some prompts and re-thinking the caller experience, it is often possible to rejuvenate tired IVR applications. Having said that, you need to be a little careful. If there are a lot of power users, who just press a series of DTMF keys, without listening to the prompts, in order to get to a particular point in the IVR tree, then you may need to keep the basic option-layout the same. Another possibility would be to play a prompt at the start which has no barge-in, which informs the caller that the IVR application has changed. Typical examples of such IVR applications are pre-paid mobile top-up, basic call-centre call routing, etc. In general these applications are more common in North America than they are in Europe (as are speech recognition applications).