"That said, how do I know when it is time to go from concentration to insight work?"
What is your current meditation practice?
In the traditions Ive been exposed to, concentration work is done at the start but its not a long time before they tell you to do insight work..I.e. in the Lay zen center, they make u count breaths up to 10, but after a few sessions, you already do Zazen which is insight, or in Goenka Vipasyana, you do concentration for 2-3 days, then do insight. I think its better illustrated like what you said "its more like a shift" that you can do even within a meditation session.
Some advice to use 1/3rd of the initial meditation time to do Concentration and the other 2/3rds to do Insight. Some also prefer to use all the time to do just insight. I do not know which approach is best but it also depends on the meditator. If you feel that your concentration ability is already strong, then you can try full insight, and if you suck at concentration (like me), then it may help to do more concentration.
But If you say you've been meditating a couple of years and have hit the A & P, then I'd think it'd be OK and about time to dive into Insight practice head-on will full force!