(just realised my new email address would be rejected from the list -
apologies if this came through twice)
If I read this correctly, you would pay.
As you state that it was neither your fault nor theirs, I take it that
it was not a bug they fixed - therefore it was a consulting project.
If you then upgrade your system to change the environment whereby
their application no longer works, then you may need to ask them to
perform another change to allow the program to continue working in the
new environment - and you would pay for that.
I relate this to the following scenario...
* XYZCo has a product called MyProg.
* YourCor purchases MyProg
* YourCor finds that due to their particular environment or way of
doing things, they need to make a change in MyProg. They hire XYZCo to
make that change and pay accordingly.
* Time passes
* YourCor upgrades to the very latest version of MyProg.
* YourCor would again hire XYZCo to make the needed change and pay