For me it requires a lot of ground training, to be able to program the FMS in flight, in a one pilot cockpit, while still aviating and navigating. It also involves understanding the altered clearance first hand, to look it up and reference it in charts. Even briefing in advance for possible changes, knowing alternative approaches.
I am still not able to really do it especially in VR - it is really hands full, I always hope there are no changes issued to my flight plan.
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