Well, don't remove the seed mix until you are ready to start feeding him something else. I, like Wolf, also feed gloop (that's the name of the cooked whole grains, pulses and veggies dish he described and, if you look it up, you will find several threads with recipes in them from the easy supermarket one to the more complicated one) to everybody. The trick to switching a bird to a healthier diet is timing, presentation and persistence. If you take out his seeds after he goes to sleep and give him a 'starting gloop' (whole grains cooked al dente mixed with beans -meaning no veggies yet) mixed in with just a sprinkle of his usual seed mix in the morning, he will start by picking the seeds off but, in a day or two, he will start eating the whole grains, too (you will find little empty 'skins'). After a few days of this and when you see he goes for it without hesitation, you start adding veggies to it, one by one and VERY gradually. I always recommend starting with sweet corn because, from the tiny finches to the macaws, I don't know of a single bird which doesn't like it. Then you would add chopped broccoli or the peas and carrots and so on and so forth. I have transitioned hundreds of birds to gloop and have never had a single one that did not eat it. As to fruits and veggies, it's, again, a matter of persistence and presentation. Some things like, say, blueberries are just a matter of persistence (I had a gray that took 5 years to try her first blueberry) but with things like carrots, it's also a matter of presentation. My budgies will only eat the raw ones if I stick a whole one between the bars like a long conical perch but I have a gray that will only eat them if they are coarsely grated. I don't feed my birds any people food. I don't know about you but I eat crap compared to what my birds eat

So no mashed potatoes for them but they do get organic potatoes. I usually buy the fingerlings that come in a bag and in different colors: purple, yellow and white and without peeling them (all the goodness is in the peel), I 'bake' them in the Potato Express microwave baggie, cut the larger ones in half and drizzle extra virgin olive oil on them - they love them! (and I do too so I always make some for me and save them until dinner

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Eating breakfast (it's the main meal for a bird) with them helps and, like I always say, raw green beans might not be what you would call my favorite breakfast (I actually hate them!) but I always eat one or two with them.
Conures are good eaters compared to other species so it should not be too hard to switch yours, it will just take time and a lot of produce ending up in the garbage but, if you keep at it, it will happen.