[Exim] Debian Light to Heavy

Hi

I currently have exim4-daemon-light installed and it works great.
Now I want to use clamav. I read on the list, that exiscan is included in
the heavy-packet for debian (sarge).

Therefor I'd like to switch. Can I easily uninstall the light and install
the heavy-packet?

What is the difference? (I havn't found this on the web.)
Do I need to change my configs? (Apart from av-integration)
Do I need to backup my config prior to switching?

Any suggestions, plase ?

Thanks,

Steffen