Helitubbie
If this is the operator I think it is, quite a number of their pilots retire at 55. Although some recent case law has clouded the issue of retirement age, the company is not acting illegally or improperly in rehiring these pilots after they have retired. There is also nothing wrong in changing their terms and conditions provided that the pilots agree to the new contract.
If my assumptions are wrong, please see answers already provided by others
RHRP