ICAO could care less if Mesaba uses their flightplan format or not. The FAA is going through "the change" and in order for our fancy jets with RNAV capability to take advantage of RNAV procedures, we need to use the ICAO format to realize the maximum benefit in the FAA's new ARTCC computer system being installed in a Center near you. What's good for the jet, is good for the Saab too, so we are all on the same page.

There is a nice PPT presentation on MyMesaba Flight Ops Homepage if you want some additional background beyond the memo and bulletin Thankfully the PPT is not required training. It will put you to sleep.