Upload & Sell: Off
A recent Lens Rental Article indicated that some of the newer lenses performed significantly better on the newer bodies. Maybe Canon are simply applying their new engineering to allow the lenses to work "even better" with the increased capabilites of the newer bodies.
(I have a 300 F4 IS and it produces great images)
I read that article several weeks ago. While I have no question that the technology improvements allow for more consistently accurate AF (and I would be surprised if it was otherwise), I have to wonder, considering how well other recent cameras and lenses still perform, about whether the magnitude of those improvements is such as to make any meaningful differences in real world shooting.
The Canon marketing folks have to really love internet forums because, when people spend thousands of dollars on new gear, they are inclined to post on internet forums with over-the-top praise for that new equipment. When they make comparisons between the new equipment and the equipment that it replaced with terms like "blows away", it makes others feel like they absolutely have no choice except to upgrade to the new version.
After having spent most of decade buying every new camera body shortly after it was introduced, I finally came to the conclusion a couple of years ago that upgrading every piece of my equipment every time the opportunity presents itself is akin to selling off a 2011 vehicle and replacing it with the 2012 model because the manufacturer added an extra mp3 jack to the otherwise unchanged 2011 model. I began to realize that my 3 year old 7D, my almost 3 year old 1D Mark IV, and the lenses I have are doing everything I could possibly ask them to do, and that, if my photography was going to improve, the main thing that needed to be done is that I needed to grow and improve my skills as a photographer.
Finally, I'm quite confident that I could post some photos taken with the 1D Mark II that owned 7 years ago and my ancient (by today's standards) original version 500/4 and, if I lied and said that they were taken with a brand new 1DX and 500/4 II, the post would generate many replies in which people were ooh-ing and aah-ing over how great the 1DX and 500/4 II combination had performed and how that combination blows away, in terms of performance, all previously made equipment.