The kits from the PES 2018 demo have been mined a few days back, so I decided to make a little comparison to show the differences between PES 2017 and 2018 kit files.
Based on the unchanged Brazil national jersey, you can see that the designers put a lot more detail into minor things like seams and waistbands, but the important changes are a) the shorter sleeves, and b) the tighter socks.
Both of these need updating to use older kits in PES 2018 without having the sleeve hems and sock details cut off. Fortunately, both of these changes shouldn't pose much of an issue for kitmakers.
Hope that helps some of you to prepare for the new season!