substitute for independent variable in 'diff

I'm being driven nuts by something that should be easy...

I have a simple differential equation that can be
un-dimensionalized by defining a new timelike variable tau=t/u,
or t=tau*u  where u is a constant characteristic time.

So that, starting from an equation that includes terms like 
'diff(y(t),t,n) where n=1 or 2,

I want to make a substitution of (tau*u) for t, and let the new
factors of u that appear through the chain rule propagate through.
  Explicitly, for n=1, a term like

'diff(y(t),t,1) should become

diff(y(tau*u) /u, tau,1) 

or

 1/u *  diff(y(tau*u),tau,1)

when evaluated.

Substitution into the first arg of diff is no issue, but  diff won't
evaluate unless the second argument is a single variable, not an
expression. Even such a simple one.

Is there an automagical way to produce this effect?

I looked at pdiff but the examples are complicated enough that I
haven't figured out how to apply it to this case.
In any event, the manipulation I'm attempting to do is so simple that
there must be a clean way to do it with the standard maxima
primitives.  I've grepped through the digests, of course, and found
nothing on point...

Please respond directly to my email (shorne at energetiq.com) --

(A separate problem I'm having is that I've stopped receiving email
from the maxima list,  <maxima at math.utexas.edu> - though I seem to
still be subscribed - probably an issue with spam filtering at my end.)

Thanks,
Steve