I suggest that after you update your sources list you 'apt-get update && apt-get upgrade apt aptitude'
Once you're running the lenny version of aptitude, then perform this:
From what I've read this should perform a minimal upgrade. I performed this during last week on my own etch system and this got pretty much all my basic services upgraded. After that perform the normal:
This should update the rest of your system.
A word of warning if you're anything like me. My exim4 config was seriously out of date, and I was unable to run it without accepting the new config and porting my existing config across. Everything else was pretty smooth.