Board Thread:LotR Mod Technical Support/@comment-29253515-20200115163047/@comment-27219153-20200117010626

The Middle-earth dimension is labeled MiddleEarth. Utumno is Utumno. DIM-1 is the nether. DIM1 is the end. DIM10 - DIM21 are dimensions created by some other mod(s) you had/have.

The Minecraft Region Scanner that I linked above is a program that can convert the ids for you automatically. You could also use it to convert entire worlds so that each world you have contains the same level.dat and then after that you could transfer builds without converting at all and they would have the correct blocks.