Update your workspace to the forge / gradle of your choice
Good video by Lex Manos on updating here:
Then start updating the mod and ask questions when you run into problems.
Betwen 1.8 to 1.11 there were some changes with ItemStack as well as some other things that were always the same for such a long time. If you have your mod in 1.8, you should be semi-up-to-date with JSON files etc. I'm not sure exactly with the jump entirely what you might run into. But you want to at least get the gradle / forge updated and working so you can go forward with starting code changes.
Rollback Post to RevisionRollBack
Mod Author and Owner of Blockhole
Owner of other discontinued or status frozen work: LimpCraft2, LimpCore, InventoryCalculator, VillageTech, Bitto'Color.