This is related to my recent question Can I weaken a coil spring consisting of spring steel?. The answer here could create an additional answer there, but I do not expect that. The difference to that question is that this is about changes in the actual spring properties, so much that there could be a fully theoretical answer with no relation to empiric coil behaviour.
What changes in the properties of a coil spring if I stretch it so much that it causes plastic deformation, and cut it to restore the length?
For example, I take a steel coil spring with a length without load of 100 mm. I extend it so much that it plastically deforms, and has a new rest length of 150 mm. Now, I cut the spring to the length of 100 mm. If it had 20 windings before, it now has about 13 windings. It does not need to be a steel spring, if a more homogeneous material simplifies it. An answer could even use an abstract material that may or may not
What is different?
How have the spring properties changed?
The mass is reduced to 2/3, how does that change the resonance frequency? Is it influenced by other factors?