A direct dist-upgrade or similar from Debian to Ubuntu won't work, at least that's my experience.
I recommend doing a clean install of Ubuntu 10.04 and then migrating the data and configuration files manually. This obviously means you need the new machine up and running before you can retire the old Debian servers. Do you use virtualisation? That would make it easier.
With Bind it probably means just copying /etc/bind over to the new machine. For the PXE server (with which I have no experience) and Trac+SVN you need to copy or recreate the configuration files (e.g. /etc/apache2) and then move the real data, i.e. the repositories, Trac instances and TFTP storage.
As to the Plone farm I have no idea but beware of Python version differences.
I have performed such migrations from Debian to Ubuntu several times and that's in my experience the cleanest and least surprising way to do it.