I want to see what are your view points on this issue. Games that have originally been 2D games Sonic games, Final Fantasy games, Mario games. IMO I believe the graphics have improved in the the games when being upgraded to 3D. However the game play has not improved at all, actually I believe we get a shallow gaming experience.

 Its a known fact that the original 2D Sonic games were fun and had great gameplay but the new 3D Sonic games suck so bad that they are painful to play. The Final Fantasy series transition from 2D to 3D has been a major letdown for this popular series. Final Fantasy series peaked with Final Fantasy VII in 2D on PSX but since then it has gone down slowly with the previous two Final Fantasy games XI and XII, both in 3D have been crap gaming experiences. Mario games I know I will get arguments here but I believe the 2D Mario games are clearly better than the 3D Mario games. The 3D Mario games have great graphics, but the fun gaming experience is not as present as the classic 2D Mario games. 

So how do people  view the impact of 3D upgrades have had upon classic 2D series?

Games have changed a lot over the years. The 2D shooters look crap compared to today's modern 3D shooters. Well platform games might of declined with the 3D upgrade.

2D and 3D games are completely different its hard to compare. 

Depends on the game/genre...etc.  Some concepts work better in 2D.  Some concepts work better in 3D.  Some concepts transform into something entirely different in the transition. 

"There is no absolute X is better than Y."

Clearly Y is better than x

OT. Do you think Street Fighter would work in 3d. No

Whether a game is good or not doesn't have anything to do with it being 2D or 3D. It's just about how much time and effort the developers put into it. Some games are good and some are bad, that's just a fact and can't be helped. Yes, there are a lot of great 2D games, but there are tons and tons crappy 2D games as well. Just like there are great 3D games and tons and tons of terrible 3D games.

Mario 64 and Galaxy are better than the 2D Mario games IMO. Don't know about Sunshine though.

Mario 3d = success

Sonic 3d = fail

Zelda 3d = success

Donkey Kong 3d = fail

Final Fantasy 3d = fail

GTA 3d = success

sports games 3d = success

racers 3d = epic success

fighters 3d = fail

...

So what about the Final Fantasy games? Will Final Fantasy XIII be a huge let down. That game has not even released, seeing how the last two games in the FF series has bombed it could be unlucky 13.

XII has set a bad precedent among a lot of gamers and the belief that the Final Fantasy franchise is going downhill seems somewhat widespread.  Combine that with all the hype this game is getting, it looks to be setting up for a virtual tidal wave of anti-hype backlash.  Good or bad, prepare for a lot of "Best game EVAR!" as well as "I expected way more, that was lame" comments in the future.