I haven't heard anything to suggest career SES members aren't like all other career civil service, but at a higher pay scale. If Obama gave them special un-fireable status by an EO, Trump can reverse that with his own EO and then they are fireable again.
I guess Obama "packed" the SES by creating more positions there and then filling them, same as FDR packed the Supreme Court. But then as a last resort (though it would be terrible management and Trump wouldn't want to do it this way) Trump can create even more slots, fill them with his own appointees, and then put them in charge of things and give Obama's people jobs (high paid, with pension) counting paperclips.