I had built a world in 1.8.8 and this morning changed my profile to open up in 1.6.4. I spawned into all-black so I quit, changed back to 1.8.8 and went back in. It was still black. I flew up and noticed I had spawned in water. That must have been what happened in 1.6.4 too. Now it looks like the land topography is the same but it's been flooded. Everything built above the flooded tiles is just GONE. What happened, and is there any way to fix it and get it back to the way it was?
Oh also, the dogs seem to be doing the butt-walk which is really weird.
Unfortunately, your world is permanently corrupted and unless you have a backup there is no way to restore it; what happened is that 1.6.4 does not recognize the way items are stored in 1.8+ and every chunk that was loaded with an invalid item (on the ground, in chests, held by mobs) was regenerated, using the 1.6.4 terrain generation algorithm, hence the appearance of squares of ocean or mismatched biomes. If you look at the game output or crash report (sometimes this crashes the game) you'll see errors like "Corrupt NBT tag / Class cast exception: ck cannot be cast to cj".
As noted there, 1.7 handles this more gracefully, chunks are not reset, but items are still lost. Also, this only affects loaded chunks, which are the spawn chunks and chunks around you out to your render distance (10 chunks in 1.6.4, regardless of render distance), so the rest of your world should still be intact.
As for the dogs, they lost their "owner" since 1.8 uses UUIDs (something like 5ew667e8w6...) while 1.6.4 uses player names (e.g. hazelluvr3, if that's your Minecraft name), and the UUID data was stripped away when loaded in 1.6.4; not sure if you can fix that (retame them?) without an NBT editor or backup.
To prevent this from happening again I highly recommend making a new profile for 1.6.4 (I assume you had 1.6.4 selected because you were using it in a different world) and change the game directory (by default the .minecraft folder) to something else so it is more difficult (unless you change the version used) load worlds with the wrong version (loading a 1.6.4 world into 1.8 would not cause any issues but would make it permanently incompatible with 1.6.4 since 1.8 automatically converts item IDs to the 1.8 format). Also, instead of selecting "use latest version" select 1.8.8 for your 1.8.8 profile so updates don't take you by surprise; many people tried downgrading from 1.8 due to various problems (e.g many people had performance issues) and ruined their worlds as a result. As always, backups are a good idea in any case.
I had built a world in 1.8.8 and this morning changed my profile to open up in 1.6.4. I spawned into all-black so I quit, changed back to 1.8.8 and went back in. It was still black. I flew up and noticed I had spawned in water. That must have been what happened in 1.6.4 too. Now it looks like the land topography is the same but it's been flooded. Everything built above the flooded tiles is just GONE. What happened, and is there any way to fix it and get it back to the way it was?
Oh also, the dogs seem to be doing the butt-walk which is really weird.
Unfortunately, your world is permanently corrupted and unless you have a backup there is no way to restore it; what happened is that 1.6.4 does not recognize the way items are stored in 1.8+ and every chunk that was loaded with an invalid item (on the ground, in chests, held by mobs) was regenerated, using the 1.6.4 terrain generation algorithm, hence the appearance of squares of ocean or mismatched biomes. If you look at the game output or crash report (sometimes this crashes the game) you'll see errors like "Corrupt NBT tag / Class cast exception: ck cannot be cast to cj".
See this issue: MC-30940 Game crashes and deletes chunks when encountering invalid items, such as lit Redstone Lamps broken with Silk Touch
As noted there, 1.7 handles this more gracefully, chunks are not reset, but items are still lost. Also, this only affects loaded chunks, which are the spawn chunks and chunks around you out to your render distance (10 chunks in 1.6.4, regardless of render distance), so the rest of your world should still be intact.
As for the dogs, they lost their "owner" since 1.8 uses UUIDs (something like 5ew667e8w6...) while 1.6.4 uses player names (e.g. hazelluvr3, if that's your Minecraft name), and the UUID data was stripped away when loaded in 1.6.4; not sure if you can fix that (retame them?) without an NBT editor or backup.
To prevent this from happening again I highly recommend making a new profile for 1.6.4 (I assume you had 1.6.4 selected because you were using it in a different world) and change the game directory (by default the .minecraft folder) to something else so it is more difficult (unless you change the version used) load worlds with the wrong version (loading a 1.6.4 world into 1.8 would not cause any issues but would make it permanently incompatible with 1.6.4 since 1.8 automatically converts item IDs to the 1.8 format). Also, instead of selecting "use latest version" select 1.8.8 for your 1.8.8 profile so updates don't take you by surprise; many people tried downgrading from 1.8 due to various problems (e.g many people had performance issues) and ruined their worlds as a result. As always, backups are a good idea in any case.
TheMasterCaver's First World - possibly the most caved-out world in Minecraft history - includes world download.
TheMasterCaver's World - my own version of Minecraft largely based on my views of how the game should have evolved since 1.6.4.
Why do I still play in 1.6.4?
Do not go backwards in versions for the reasons stated above.
My Github ด้้้้้็็็็็้้้้้็็็็็้้้้้็็็็็้้้้้็็็็็้้้้้็็็็็้้้้้็็็็็้้้้้็็็็็้้้้้дด็็็็็้้้้้็็็็้้้้้็็็็็้้้้้็็็็็้้้้้็็็็็้้้้้