Blackhawk9
I have no inside line on this but I would not consider that simply replacing a twin 355N with a single 407 was not wholly down to manufacturer support issues. After all an air force is more likely than other operators to carry significant spares.
Perhaps the relative complexity and consequent drain on engineering capability of a twin [perhaps made worse by the spares supply issues raised above] was a significant driver to move to a single. If it were not a single/twin issue they might perhaps have been expected to take on 427's.
If the 355N's have been withdrawn have you any idea where they were sold on to?