In the README file (in "Source Location"):
If a system is being redefined, the top-level pathname
- changed, if explicitly supplied or obtained from *load-
- changed if it had previously been set from *default-
- left as before, if it had previously been set from *load-
and *load-truename* is not now bound
There is a minor wording issue, *load-truename* should
always be bound, but it might be NIL.
If it is NIL, asdf will try to apply TRUENAME to it. That fails
in, e.g., MCL. The README suggests that it should be ignored
in this case and the old pathname kept.