ValhallasAshes: I can't help you with Flawless Widescreen, but I'll point you to an alternative method that worked better for me, you could try:
https://www.gog.com/forum/star_wars_knights_of_the_old_republic_series/widescreen/post111 I hope it helps.
gunterb: Thanks, that actually went pretty smoothly... kinda. The game's resolution I was able to change, 1920x1080 appears now as an option on the graphics menu and it works. Game run, no freezes or any other issue.
BUT my HUD is still 800x600. Tried with both the updated and the regular one, and even another resolution, but I didn't manage to get it working.
Thank you so much for the help either way, even with the small HUD it still is way better, just some getting used to if there is no other fix to this!
You might have missed it in the instructions. But when it comes to the HUD files. Once you download the zip file, you need to extract the files. Often with a lot of zip extracting programs, they will extract the files into a folder of the same name as the zip file. So what you need to do to get it to work, is you need to copy the files that are inside the folder (not the folder itself. the actual individual files that are inside the folder) directly into the "override" folder in the game files. If you copy the zip file or the folder it creates upon extraction into the override folder, the HUD correction will not work. It has to be the actual files.
I've attached a screenshot to this post to show what it should look like in your file explorer.
(Note: In the case of my install, the files in that folder are labelled 800x600, but those are actually the 2560x1440 HUD files. My best guess is that those files are overriding the 800x600 HUD files the game looks for. So are labelled in a way the game can understand even if they are for a resolution that is different from what they are labelled. I didn't make them myself, so I have no way of knowing the reason for why they are labelled the way they are.)
Hope this helps.