I tried out pkg_rolling-replace(1), and it failed because some packages had
  a spurious space in the path. Looking closer, I found

  Now, I can reach in and remove those, but would anyone know what inserted
  that space?

No, but I have found that once I cleaned this up it does not recur.  So
I think the extra space bug is from long ago and not present in current
pkgsrc.

I just committed a change to pkg_rolling-replace (0.11) that should
strip the leading spaces.