Unfortunately, no “general”, simple, automatic process is likely to succeed at this task. See “How does LaTeX relate to Plain TeX” for further details.
Obviously, trivial documents will translate in a trivial way. Documents that use even relatively simple things, such as labels and references, are likely to cause trouble (Plain TeX doesn’t support labels). While graphics are in principle covered, by the Plain TeX
Translating a document designed to work with LaTeX into one that will work with Plain TeX is likely to amount to carefully including (or otherwise re-implementing) all those parts of LaTeX, beyond the provisions of Plain TeX, which the document uses.
Some of this work has (in a sense) been done, in the port of the LaTeX graphics package to Plain TeX. However, while graphics is available, other complicated packages (notably hyperref) are not. The aspiring translator may find the Eplain system a useful source of code. (In fact, a light-weight system such as Eplain might reasonably be adopted as an alternative target of translation, though it undoubtedly gives the user more than the “bare minimum” that Plain TeX is designed to offer.)
This answer last edited: 2011-05-30