[Zope] Where's Data from Instances/Installations???

[Harry]

Alright, I figured out that products like Squishdot have the DTML files of
their basic structure in the Products sub-directory of the zope folder.
Changes made there will appear in NEW instances of these Products.

My question, though, is where is the data for EXISTING instances kept, and
is it changeable?

For example if I create a Squishdot instance called MySquishdot, and at a
later time I want to change some of the look and feel of built-in interface
elements, where do I look, and can that even be changed?

So far, my best guess is that ALL instances and their data are kept in the
Data.fs file in the var directory - while a nice central location, it does
seem to preclude any direct changes at a later date.

So, how the heck do I make such changes, short of making them in the
main/original installation, create a new instance of the site I need to have
changed, and then export and import all needed data (and how do I do
that??).

Overall, as some quality feedback, if that is the only way to change
existing instances, I'd have to say that this is a MIGHTY cumbersome way of
working, because even if you are terribly organized and have everything
fleshed out, there will always be changes as you discover new ways of doing
things... then how do you upgrade the easiest way?

Harry