How complex is it to update a FM6 application, created in Developer, to
FM8? The customers don't have a massive amount of data, but I'd like to
automate the upgrade as much as possible.

Ideally, they install the new app and it would import the data.

- Scott

Getting the data out of 6 and into 8 is not the problem.
Making an application written in 6 work in 8 is the problem.

If your saying you've already got the new app written then you could
script the entire process of importing data.

Get a copy of the client files.
Convert them to FM8
Add table references in your solution to the converted files.
Using your solution, go to a layout displaying data from the converted
file and find all records.
Do an import selecting your solution as both source & destination file.
If you get all that working then all you have to do is script the
conversion part; use the "Convert File" script step for that.