upgrading package - configuration file

[Sven Burgener <svenb@bluewin.ch>]

Hello

Can anybody tell me what happens when the config file format of a
package changed from one version to the next.

I mean, in the process of downloading, unpacking and installing the 
upgrade, what really happens?
Are diffs used for this (config files)? Is this all up to the package
maintainer?

Sorry if these are trivial question. My assumption is that both, old and 
new configuration files will be around, ready for the sysadmin for merging,
correct?

Cheers
Sven