Hey Uzi15,
Working for a large automotive company in Canada, we repurpose/retask old robots all the time. Because we do this so often, you are correct, the money saved in repurposing a robot seems relatively low when looking at the scope of the whole cell project. However, from my experience with large companies, because of the amount of times they do these cell upgrades that "relatively low" saving becomes quite substantial.
As far as the process of retasking, for us, it begins with an appraisal of the current system. Controller, drives, robot hardware etc are all assessed to be allowed to be repurposed. The last thing you want to do is commission a new cell only for the robot to last a few months before it needs another overhaul. This is where we have been sending our robots for refurbishment. This can be costly. Depending on the condition of the robot/controller, this can almost cost as much as a new robot. I would recommend going with a trusted 3rd party robotics company to do the refurb instead of going with the manufacturer to save on cost. I repeat, ONLY IF THEY ARE A TRUSTED COMPANY. In total this process can cost anywhere from $0 (Considering the robot is in perfect condition) - $35k (For refurbish cost). Consider you spend $25k opposed to $50k for a new robot and controller and you repeat this for 100 cells over time. Eventually it becomes very profitable. Saving tens of thousands of dollars over time. Eventually, robots will have to be replaced but this kind of "recycling" I have found to be prevalent among larger companies.
Cheers.