IF (and that's a big if) 4J changes nothing about how worlds have been generating... if you leave areas unexplored and then explore them after the update, the terrain in those areas will follow the new terrain generation formulae and generate in those areas as if the seed itself were created with the latest update. So, for example, right now, the seed generates an ocean biome in an particular area but that same seed would place a jungle in the same place if created post-update, then jungle terrain would generate if the area were unexplored but not overwrite the ocean terrain if the area had been explored... however, regardless of whether or not the area was explored, animals would generate in accordance with the new biome... so you could get ocelots spawning on small grass islands in what looks like an ocean biome but really isn't anymore.
Personally, I recommend generating a new world with every update. There are ALOT of significant drawbacks to trying to get older worlds to work well beyond a big biome-adding update. Biome shift causes the weather and animals spawning to not match the terrain and there are no guarantees that the seed would generate the terrain you want in the areas you've not explored. You might just get more forest if that's what the seed would put in that area post-update if you generated the same seed from scratch. Also, the transition between various terrains can be really brutal (e.g. sheared off mountains right at the chunk boundaries)
If you do still plan to do this, be sure to fully explore the area around where the End Portal is located... or else you can inadvertently wind up with the world that doesn't have one (if the portal moves locations in that seed post-update). Personally, I now make a habit of fully exploring every world I open before I do anything else. That way, at least that seed has all the features it should have relative to the update it was created in.