Upload & Sell: On
I don't think think there is any hard rule, and you won't get any statement from Canon on it. It's just that those made prior to 2006 may be more prone to the good-copy, bad- copy syndrome, with the proviso that Canon can often correct a bad copy when sent in. I don't believe Canon have made any design or material changes to the 100-400, however since 2006, in keeping with the more and more demanding camera technology, it would appear that Canon have commensurately tightened their manufacturing and QC standards, making the newer copies more consistent with the original design quality.
My first 100-400 copy bought in 2005 was ok, but I never felt it was as good as it could be, or as good as other copies I knew about. Having sent my copy in for correction three times through CPS, to no avail, earlier this year I decided to sell the old copy and buy a copy from the latest production, and yes, now I feel I have a copy that is as good as it can be, which is excellent.