Lets say I install a Minecraft mod which adds new blocks and items. I then update to a newer version of Minecraft, which does not have these new blocks.
How does the game handle maps with what are now unrecognised items in inventory, unrecognised items in chests, and unrecognised placed blocks?
The world will just generate with air blocks replacing non-vanilla blocks, and your non-vanilla items will just disappear. Re-installing the mod will not bring the blocks or items back, unless you do so before loading the world again.
(As a side note, some mods used to render your world unplayable unless the mod was installed again, I'm pretty sure Notch fixed it though, but there might be a few that mess things up.)
The mod will make new item IDs, and items are stored by the item IDs. So, for example, I once had the portal gun and I updated the mod and I had a melon stem in one of my chests :). Any other items or blocks are just not there!
When I started to use a different minecraft version, it actually turned the blocks into what I guess must have been the closest texture. I think that the game gets confused and thinks that you're looking for (block thats texture Is most similar) and turns it into that. For example I used a type of glossy stone block, and when I changed versions, it turned into stone. The slime block turned into, well, a block from mo creatures of all things lol.
This is maybe what happens in later and more developed versions.
