[plt-scheme] Parameterize and the GUI

Anyone know how to change parameters, while using the GUI? Because when
I try to do it I'm like...

(parameterize
  ((foo something-new))
 (um...)
 (kill all the GUI windows)
 (shut down everything)
 (start up a new GUI))

Or else I just say (foo something-new) and treat the parameter like a
global. I'm not sure (parameterize) can be used right with an
asynchronous event loop type framework. You could nest event loops, but...

Maybe I'm just not thinking with the right structure. How would you
structure a GUI program, where you called something that never returned
but put the GUI into a different state based on the new parameter settings?