I just examined a set of 133 Scheme source files of from guile version 1.8,
and every one of them is "well-formed".  That is, the sweet-expression reader
interprets them the same way as an s-expression reader.

To confirm this, I used the new "diff-s-sweet" tool
(in the readable project "develop" directory) and ran:
 find /usr/share/guile/1.8 -type f -name '*.scm' -exec ./diff-s-sweet -P -S -w {} \+

I think that's great evidence that sweet-expressions are very backward-compatible
with traditional s-expressions.

 --- David A. Wheeler