(A convert-ly rule may be needed… maybe in any case to
change whiteout to whiteout-box? ...or maybe not?)
Looking at the existing uses of \whiteout Xxx.whiteout, it does seem
best to leave the plain box whiteout in existing scores. Convert-ly to
\whiteout-box one way to preserve the behavior, but it would be better
to leave the old markup function \whiteout for the old behavior, and add
\whiteout-outline.
If I may chime in here: I think this is unnecessarily conservative. The
new method for whiteout is certainly much more elegant and more what one
would expect in the first place, than the method we have been using.
It’s rather going to be special cases in which \whiteout-box would be
preferable, and in most cases it will be \whiteout-outline, not the
other way around. \whiteout-outline is just a bit too clumsy to be used
for what is a real improvement.