My old vector files sell ok still, but there is a constant decline  . I uploaded then in 400x400 preview stile on ss so now i m stuck whit the small image . Is there a way that i can upload a new bigger preview , without loosing the seniority of my files  ?

Nope  .  As far as I know it is not possible. I wanted also a few months ago when I started to make images for the New Year 2017 and I wasn't able to do it. I find it stupid, because in some cases a lot of images will become obsolete in time (like those containing a "Happy New Year 2015, 2016" message). Moreover, contributors tend to delete them and the site gets a lot of 404 Error and it's not good. But how I am to judge