Making slightly different objects...

 From:  JTB
I make a curve and I use extrude .. Then I use "show points" and make some changes... Everything is working ok.

After that, I pick both objects (curve and solid) and copy them to an other place because I want to change something. The solid doesn't move separetely from the curve.
The problem is that now, the new solids are not connected to the curves as they should be (since they moved together).
As I understood from an older post of mine, moving the solid breaks the connection with the curve that created the solid and there is no update.
I can't understand why it doesn't work now...

My purpose is to be able to make generally similar objects but with some small differences. What is the method for that?
I can imagine that copying the curve and then using extrude is the simple solution but what if there are 10-15 steps before copying?

Also, the connection is lost when I use boolean operations... For example I make a cylinder. I make a smaller one (not in the center so shell command is useless) and I use boolean->diff. to remove the second. If I want to change the shape of the big cylinder I can't. Why? nothing moved, but the curve is now useless.