Yes focusing screen can be replaced. You just need to buy the type of surface required.
The only things that self replacement is not possible is the mirror and the prism after the focusing screen. So when replacing, be careful and never to use sharp objects to pry them to avoid scratching the mirror and prism.
It get scary with first attempt replacing the screen, make sure minimal surface contact, gentle removal of the screen and ensure dust removal before putting it back.
for your quesiton, no it doenst affect the image/ focusing. but it can be quite irritating to see the scratch everytime you take a picture. Anyway, I scratched mine before. Can refer the my previous thread for replacement info.
Only if the scratches are bad enough to affect exposure readings (diffraction) as well as AF performance. IIRC, almost all AF systems that use light passing through the focusing screen and I've not heard of metering cells being located in the mirror chamber in modern DSLRs.
Other than that, no, slight scratches would not affect Image Quality (IQ).
Changing a screen sounds scary at first, but once you get used to it, it can be done in a few seconds on the train or with a little more difficulty, while walking down the street.