Changes in the listing: main image, secondary images

Hi guys. Bradley, if you can read me that would be great.
I'm considering changing my main image a bit to improve it, but my product is selling for more than a year now... is there any problem if I do that? will I lose a lot of ranking or mess up my sales too much?
I read somewhere else that it was forbidden to change the main image.....
I want to do the same with the other images, since I have all the same ones since the beginning.