I'm in the keep what you have camp. I have thought about this with my truck as well which is pretty much only used to tow our fiver and take the odd load to the dump. A 10 year old truck may not be as economical as you think by the time you completely service it and do any necessary repairs to it (my past F350 cost me over $5k in the last year I owned it with front end repairs, brakes, axle seals, etc.). And this is without considering any other surprises that come up; at least you know what you have with your current truck.