Unfortunately, this is how life works. There is not much that we can do as I am sure that PM used some hidden clause in the fine print to force this change onto its customers. Options are either to leave PM and look for other good subscription offers with other providers or to just live with it (in case this offer is the best in terms of data/talktime vs cost).
I am not affiliated with PM in any way but if I have to think in terms of whether it is ethical for the company to have done so then this is what I think. As customers, don't we all do something similar if it suits our requirements. For example, if I get a better offer with another provider, I am going to move right away without bothering to think about PM's bottom line or how losing a customer is going to affect PM. So, in all fairness, PM should get a similar opportunity to act in their own business interest.
