The colours in a GIMP image can be modified by using the curve tool. When I apply the curve tool, the original values are forgotten. Is is possible to apply the curve tool as a layer filter so that the curve that I am using as a filter and the original image pixel values are stored for later modification?
Sadly, no. In Photoshop, this is called "adjustment layers" and it is one of the key photographic features Gimp currently lacks.
We have a question about how to work around this, here: How to create the equivalent of an Adjustment Layer in an editor that does not support it? (but I'm afraid the answers aren't going to be very helpful).
You can find "filter layers" on the Gimp roadmap, somewhere in the middle of the top 15 high-priority features, but unfortunately it looks like no one is actually working on it and it's scheduled for only the vague "Future".