There is not much change that the current 777 will be converted astheir wing box is to light structurally and cannot be upgraded as they did on A300's (this is from the head hungo of engineering at Boeing) The heavy weight 777 (LR etc) they are currently building could be new build as a freighter but at a new aircraft cost...
The other problem is commonality. With Airbus who are succesfull with their push into the market with the A300 possibly A330 soon and A380 they are establishing a new standard for containers to be used. At an air cargo workshop all desk drivers, beancounters and cargo staff agreed that commonality was paramount... The 777 would have the same problem as there is with the combination of MD11/747 as the are all different...
The answer? don't know..