Most likely stuff from the DM will break and some from vanilla.

This is why it's best to either back up your 1.8 DM singleplayer worlds and hope for the best or just start over.

Well, at least my new library is 99% vanilla. I should be able to salvage that at least. The Eye of Harmony's toast, though.

 Wait wait wait. There is a chance everything will be fine? Like: Everything? Why don't you... Test it. With another, smaller world. Alright, there are the buggy chunks, but still. Make a world in 1.8, get some stuff in the world and in your inventory, and start the world in 1.12.(1 or 2? I don't know). I might be wrong, but then please do correct me. I am not familiar with coding at all.

 Alright, I just thought I'd mention it. I couldn't have known you already were busy with that, because, well let's be honest, you only share interesting stuff. And testing like this is a boring thing to read about. 

Oh, well as long as block IDs are the same, I'm happy. Back to work on my single player TARDIS. Thanks Sub!

All I know is that between 1.8.x and 1.12.x no IDs changed. At least, as far as I remember. That being said, from porting worlds I have learned that there is a chance that all entity data gets cleared. That includes inventories, chest inventory and other containers lime dispensers and command blocks. Got something to do with changed NBT data system. So, if the DM doesn't change it's IDs, which is rather unlikely, everything should be fine.

 Yes!!! Finally, that troublesome period will be gone! Thank you Sub! There are so many things in the past two years that I've wanted to do that I finally will be able to!!