While I completely get where you're coming from, realistically this can be very messy. You'd quickly find that an increasing number of your support calls were from those users having issues. 15% doesn't sound like a lot, but 15% of a million people is 150,000 people.
So, you'd succeed in angering a lot of customers. Much better to try and inform all of your customers (not just corporate) of the upcoming changes, and provide support materials for them to follow while giving them a reasonable timeframe to do so.
