John,
Although not a current Chinook operator , perhaps I can shed some light. The block 1 upgrade to FADEC simply changed the Eng Fail Caption counter software from a single detection count to about 6 counts, thereby ensuring that if a fail was detected it had to exist for 6 counts rather than the spurious single. But thats all they changed. FADEC is the same. BD refused to certify it not because it was unsafe, per se, but they couldn't read the code using their static code analysis to their required standards.