I had my lens assembly replaced last summer and had a "feeling" that this was the case but comparing similar shots before and after the replacement they seemed fine. However, that "feeling" didn't leave me for months. It was more that my instinct just knew there was a difference but even pixel-peeping didn't even reveal any real disparity. However, that's not to say that I disagree with your experience at all as I'm well aware that individual examples of a camera or lens can, and do, differ greatly. Just look back to the reported focusing issues with the Nikon D800 and one of the Canon 1D models when they were launched. One thing I DID notice about 6 months after the lens replacement was the massive amount of dust on the sensor whilst doing some long exposure photography. I contacted Fuji UK and they were happy to clean the sensor under warranty. I did, however, mention in my email to them that the lans had previously been replaced by themselves. Not sure if they would have been so accommodating had the warranty actually expired [there were four days left on the warranty when I emailed them!]. Good luck with getting it sorted out and if it's Fujifilm UK you're dealing with I'm sure your experience will be a professional one, they have always been good with advice or service when I've contacted them.