Here's the same mistake, made by another famous institution.
Back in middle of 2009, the folks at Harvard Business Review moved to a new online home. They left the shelter of the business school and established a site of their own. They switched domains from hbswk.hbs.edu to harvardbusiness.org.
Good idea, bad execution.
Why? Because when they moved, they switched the old site off.
At the time, I created two charts - a before and an after - to see the extent of the damage they had caused to themselves:
Luckily, being Harvard Business Review, they recovered. But they lost important ground to their competitors. Knowledge@Wharton has far outpaced them online.
And now Forrester makes the same mistake.