As of this week's builds (since 4/10), when I open a project in NetBeans for
which I have made edits to the build.xml file that NetBeans created, NetBeans
now overwrites the file with its own version of build.xml and all my changes are
lost.

Last week it was inserting the line 

    <import file="nbproject/build-impl.xml"/>

if I had erased it, but otherwise leaving the file as it was. I didn't like
that, but I could live with it.

The build.xml files for the tutorial we are revising use Blueprints targets in
another directory. We do the following import:

    <import file="../../bp-project/main.xml"/>

This main.xml file determines whether it is being called from within NetBeans or
not, and if it is, it imports the nbproject/build-impl.xml file. By overwriting
our build.xml file, NetBeans makes it impossible to use this logic. 

But in general I would think that anyone who tried to open an existing project
with a build.xml file would be extremely upset to find that the previous
build.xml file no longer existed.