Unless the FAA grants an extension beyond the TBO, the MORE program is the only way to legally fly past the 3600 (or 3000 or 3500 depending on model) hours. It's not like a piston where under 91 you can over fly it. The when PWC certified the PT6, part of the Type Certificate was a mandate to comply with all manufacturer service bulletins. One SB specifies the TBO as well as another that specifies certain components are life-limited by hours or cycles. Those SB's, regardless of whether you are 91 or 135, must be complied with. If the engine has been run for 400 hrs beyond TBO, then there is documentation to support it. If the operator was allowed to run it like that without the MORE STC, then I am pretty sure there is another specified limit, and the ability to operate it like that remains with the operator because that operator's maintenance program and operational history was the basis for the extension. In other words, in this situation I believe when the operator sells the plane, the engines are due.