Ted Harding wrote: [...]
What are my chances of successfully upgrading from Lenny to Squeeze?
Unsure but I suspect it's correlated with how well you follow instructions! ;-)
And how to do it?
http://www.debian.org/releases/stable/i386/release-notes/ch-upgrading.en.htm...
And then what about upgrading from Etch to Squeeze?
I think I'd do it in two steps if resources permit, from Etch to Lenny and then Lenny to Squeeze. Etch to Lenny is in http://www.debian.org/releases/oldstable/i386/release-notes/ch-upgrading.en.... at the moment. If I needed to go from Etch to Squeeze, I'd read both sets of release notes and try to make sense of it.
And what is likely to happen to packages (some external) installed on these obsolete Debians? (Especially where I have made my own modifications to software, most often configuration files and the like)?
If you mean debs from external sources, nothing will happen, unless either:
1. you update the external sources.list line for its maker to point to the new release for squeeze, when it will probably be upgraded with the rest of the system;
2. some package it requires is being replaced with an incompatible one, when it will be removed unless you tell aptitude to hold it and that might prevent part of the upgrade happening.
Configuration files declared in the deb packaging get special treatment and you should be asked for what to do about changes.
If you mean tar.gz compiles put in /opt or /usr/local, then debian probably won't do anything to them and whether they work depends on whether they can cope with the new versions of things they depend upon, like libraries.
Good luck!