[plt-scheme] Automatic recompilation of source files with new versions of PLT Scheme

I was excited by this new feature of PLT Scheme version 4.2.2:

>  DrScheme now, by default, compiles all of the files that are
>   loaded when it runs a program and saves the compiled files in the
>   filesystem.  This should lead to faster load times (not faster
>   runtimes) since it avoids re-compiling files whose dependencies
>   have not changed.

But when I tried to run one of my programs in DrScheme I get this  
error message:

> ... search_sls.zo:1:0: read (compiled): code compiled for version  
> 4.2.1, not 4.2.2

I've received similar messages in the past, whenever I've switched to  
a new version of PLT Scheme.  In the past I've fixed this manually, by  
deleting all the *.zo object files in the "compiled" folders that PLT  
Scheme creates.   I was hoping that the new version would do this  
automatically.   Is there some tool for updating the object files that  
I've overlooked?

On a related issue:  I'm not really happy with PLT Scheme storing its  
object files in folders it creates within my source tree, since the  
code is portable R6RS Scheme and designed to be used with any R6RS  
implementation.  Is there some option for having the object files  
stored in some place outside of the source tree?

-Tom Gordon