Are there any changes from the original versions in the Metal Gear Solid HD Collection?

The only "pro" for PS2 I can think of are better analog button controls for the PS2 version (the PS2 has analog controls on all buttons except start and select and this was supported in MGS2, specifically for aiming and perhaps for something else).

Other than that, I think the HD version will be the better choice, first of all is the HD aspect, meaning upscaled graphics, but also worth mentioning that included are the later version of both games, Substance for MGS2 and Subsistence for MGS3, which included extra content and in the case of MGS3, a new camera option (although some features some minor features which were included in Substance/Subsistence were removed from this HD version, as well as nightmare sequence which was available on the original Snake Eater). Also included in the package is the PSP game which wasn't available for either platform before.

In conclusion, I say the HD collection all the way.

I believe just the fact that there are achievements to be made so it gives the game another angle than just going through the story, I found that to be really awesome. I don't know if I have a poor memory when it comes to the game, but it seems like there was game objects added in the game not to the story but in game additions. But again, I usually prefer the first MGS, and the fourth, but in playing the second one in HD there were some new things that did come up. I enjoy the HD version a lot and fully endorse you purchasing it for whichever console you are looking at.