[plt-scheme] Bytes/Chars/Strings/ByteStrings

I have an application I'm porting from the 200 series to the 300 series. 
I believe, for this particular application, I need to be most careful with:

(20X) characters becoming (3XX) bytes
(20X) strings becoming (3XX) byte strings

The particular application is a linker, and deals with bytewise data; 
these seem to be the most significant, underlying type-system changes 
that will effect me. From people's experience porting, were there any 
other major 'gotchas'?

Cheers,
Matt