Knee-jerk response

[Glenn S. Burke <GSB at MIT-ML>]

is approximately the best thing proposed so far.  It is extremely
inconvenient that even when one understands why you are getting
screwed it is difficult to do anything about it.
|forget-macromemos/|| should be renamed more conveniently (or another
entry should be made) and there should be something you can call to do
the mapatoms if you are super-paranoid and/or lazy.  (Note that you
don't want to mapatoms just |forget-macromemos/||, and remproping the
macroexpanded property is wrong;  you want to increment it.)  The
other thing to do is to memoize the expansion on all the dependencies,
so a particular SETF expansion would "know" that it depended on both
SETF and FROBNICATE being redefined...  But in general it probably is not
possible to back up as far as is necessary, given the case of macros
evaluated for a value which makes up some part of a memoized macro
expansion.