Well nothing new there. The Wizard of Oz 2D is rated for P while the same film in 3D is rated PG. Seriously, the hell is that about!?
AFAIK, The Wizard of Oz has a "grandfather clause" hold on it's G rating. Once the MPAA gives you a rating, that's your rating and there are no take-backs (unless you want to re-apply). The MPAA said once that Wizard of Oz was G, so Wizard of Oz gets to be G forever, even if modern standards and public morals shift and change.
But the 3D conversion is a movie that has been "tampered with" and that means it needs to run through the classification process all over again, which means it has to be judged by modern standards, not the rating that they were given half a century ago by people who are probably dead.
Also, movies apply for a specific rating, which can be denied based on a rough set of disqualifiers. Saying the word "fuck" even once disqualifies you for G, three times disqualifies you for PG-13, and saying it 400 times disqualifies you for R, but that doesn't mean that you
need to say "fuck" three times to get an R. The Wizard of Oz 3D could have applied for an R rating, and the only result would have been that the MPAA would have found no reason to refuse the movie's classification. If The Wizard of Oz 3D had applied for it's old G rating and been shot down (because standards have changed), then the movie might have been delayed, and they would have had to pay a new fee to apply again under the PG rating. I don't know if that happened to Wizard of Oz 3D, but sometimes it's just easier to apply for a PG rating if you think there's doubt that you'll get the G rating (which feeds into the change in what G means). It's doubtful that the Wizard of Oz 3D's producers saw any negative consequence from a PG rating.
People did see consequence from Deadpool's R rating (Deadpool could almost have been PG-13, but it's producers wanted just a little more wiggle room to make exactly the kind of movie they wanted to make so they asked for the freedom of an R rating, and the perceived audience loss from that rating prompted Fox to cut the movie's budget, but the producers thought it was worth it), but it did amazingly well at the box office so now people are reconsidering R ratings and that consequence has been greatly reduced.
I would say that with how standards have changed and evolved over the years, from Wizard of Oz clinging onto a G rating it apparently can't hang onto forever, to that movie NC mentioned that got struck down for having a gay kiss, to G movies becoming the bastion of un-objectionable kids movies, to the financial impact of R being reconsidered, this is all a kind of pointless debate. The MPAA isn't in exactly the right place, but they move around. Yeah it's funny that South Park can flirt with 399 f-bombs just to mock the line, but then where should the line be drawn? Or should there just not be a line? Does that line really bother people? It's just a guideline, so who cares? And I say this as a guy who opposes censorship.