Earlier quoted context omitted.
I think Hawking is now so firmly tied to that voice that he would probably never switch for public speaking engagements and the like. I could see him doing such a switch for personal interactions.
I recall his biggest qualm with his current synthesized voice is that it did not come with a British accent. :-) I'm not sure how sentimental he is but he does seem quite tied to that voice since there have been lots of advancements in voice synthesis since he originally got this and yet he's chosen to keep this one.
Should be just as possible in the present or near future.