I have been searching for a professional discussion on this topic, thank you . In my experience in Australia, the change to 'Ah' has occurred rapidly over a time-period of around thirty years. I tried as a teacher to hold on to the correct usage, to no avail. What is happening now is that media interviewers and politicians are using 'Ah' as a pause like 'Um' to think of their next point to be made. Looks like a change that won't be reversed though my long-gone English teachers would be turning in their graves over this ugly usage.