Thing is, he's already created the builds in the worlds, so the id's are already set. As long as the builds have a fairly low number of non-vanilla block types, and no tile entities, the simplest solution is to use world edit, or mcedit, to change all those non-vanilla blocks to unique vanilla blocks. Then he can move the build over and world edit them back.
Midas Gold would work, but it would be tricky. You would have to change all the present blocks in the world, and copy over the level.dat at the same time. That could be mitigated by deleting all of the chunks in the world except the ones you're interested in, limiting the number of id's that need changing.
I also like to mention the existence of
midas silver, which is a command line version of midas gold. Which is a fair bit easier to use if you're working with a large world on a remote server, or trying to script something.