IC2 Blocks are lost when transporting an older version of save to a newer version of MC

Are you sure the 1.19.2 version of the mod is really IC2? As far as I'm aware, there are no official ports of IC2 to 1.19.2 with the exception of Speiger's IC2 Classic, so the mod you are using is probably a different one like Tech Reborn or some such that seeks to imitate IC2 as closely as possible. In that case, the blocks do not load upon opening the world because the mod they come from is no longer present, even though an extremely similar mod is present. Perhaps try again with IC2 Classic for 1.19.2. If that doesn't work, then it is probably simply not possible to transfer your IC2 save file from 1.12.2 to 1.19.2.

Also, IC2 Classic is not exactly the same as what you were using in 1.12.2, which looks like it's probably IC2 Experimental. You see, first there was IC, then came IC2, then IC2 branched out into IC2 Experimental and IC2 Classic, where both have IC2 as a base but add different stuff. A good way to test if you are running IC2 Experimental instead of plain IC2 is if the mod includes something called a "Fluid Reactor", which is an upgraded version of the regular Nuclear Reactor that is substantially more complex to generate power with but can produce greater quantities of power. If you are running IC2 Experimental, then the Experimental-specific stuff (such as the fluid reactor, and probably lots of other stuff) definitely will not be recognized by IC2 Classic. However, there is still a chance that the original IC2 stuff will be recognized in the save file when using IC2 Classic, so it's definitely worth a try.

Yes, ic2 experimental exists on 1.19 https://jenkins.ic2.player.to/job/IC2/job/1.19/.

It is generally not possible to seamlessly port worlds across minecraft versions due to the numerous changes they go through when porting. Blocks and items that existed in the mod in 1.12 may not exists in 1.19 and vice versa. You're also not getting any of the new vanilla worldgen changes either. Porting worlds across versions is not recommended and will likely result in world corruption.

If you want to start playing a new mc version, especially when the difference is this huge, you'll have to start over.

Quote from Su5eD

Yes, ic2 experimental exists on 1.19 https://jenkins.ic2.player.to/job/IC2/job/1.19/.

It is generally not possible to seamlessly port worlds across minecraft versions due to the numerous changes they go through when porting. Blocks and items that existed in the mod in 1.12 may not exists in 1.19 and vice versa. You're also not getting any of the new vanilla worldgen changes either. Porting worlds across versions is not recommended and will likely result in world corruption.

If you want to start playing a new mc version, especially when the difference is this huge, you'll have to start over.

Looking at that link, it appears to still be in very early development. It states directly that it is not yet stable and breaking changes may happen with updates. Perhaps IC2 Experimental has always been this way, but I still get the feeling that this isn't ready for use yet. But yes, I do stand corrected about no official IC2 versions existing for 1.19.2 besides IC2 Classic.

Quote from krbbbb

I'm sure I'm using IC2-exp rather than the classic one. I also tried the latest version (industrialcraft-2-2.9.24+ex119-1.19.2-forge.jar) but blocks were still lost. I hope a newer and more stable version could solve this. Thanks for your help!

Yeah don't expect any Major MC version Jump to be going well for your world.
There is kinda the expectation that mods use these to refactor their code and changing things up.

IC2C for example did undergo between 1.7.10-1.10.2/1.12.2 a whole rewrite from scratch.
Same goes for 1.12-1.19 versions.

The only thing that has multi version support in ic2c is the reactor planner, which usually even gets backports with new Encoding formatts.

But I highly suggest you if you want the IC2 Experiece to try the IC2Classic version.
it has been well tested, is still maintained, and has a lot more QoL features you will most likely love.
(Including a ingame wiki)

Quote from Speiger

Yeah don't expect any Major MC version Jump to be going well for your world.
There is kinda the expectation that mods use these to refactor their code and changing things up.

IC2C for example did undergo between 1.7.10-1.10.2/1.12.2 a whole rewrite from scratch.
Same goes for 1.12-1.19 versions.

The only thing that has multi version support in ic2c is the reactor planner, which usually even gets backports with new Encoding formatts.

But I highly suggest you if you want the IC2 Experiece to try the IC2Classic version.
it has been well tested, is still maintained, and has a lot more QoL features you will most likely love.
(Including a ingame wiki)

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So would this explain why everything literally vanished from my base when I tried updating from 1.7.10/2.2.828-experimental to 1.8.9/2.3.262?

yeah it would, the blocks of the IC2 version you were using no longer exist and while equivalents do exist in the new version Minecraft doesn't know how to convert them so it just deleted the blocks from missing mods

Quote from SCC

So would this explain why everything literally vanished from my base when I tried updating from 1.7.10/2.2.828-experimental to 1.8.9/2.3.262?

Like JasonS05 said. It actually explains it.
The new architecture also includes ID changes.

And since moddevs treat them as "Uncompatible" between MC versions ids will not match and or there is no data forwards handler included.

So don't expect forwards compat.