Well, obviously anything involuntary means that there is no fare difference, right?

Unless you mean the case where you are booted from the flight you had booked, and you want to take a completely different routing?
According to EC261/2004, you can be rerouted to another nearby destination, with the caveat that the carrier carries the cost to transport you rest of the way.

I would guess that you'll get free rerouting if the cancellation is closer to the date, there is no similarly timed alternative and you want to fly to a close-by destination.

If it is further from the date, and/or you want to fly to a completely different destination, then it's not looked at as rerouting within your rights, but as a reimbursement and applying the sum to a new ticket elsewhere.