Out of curiosity, how are you currently feeding vegetables? In small chunks, whole, or processed?
I have two ways that I feed veggies that have worked to introduce them to birds who've never eaten them before. I'll give them to you in order or preference with my fav first and a possible alternative second.
First, food processor chopped portions. This is actually how I prepare my chop always. I made a post about it a while back. I get my veggies, usually a fairly wide variety, and put them through a food processor. The result is seed-sized veggie bits that I mix all together. I also mix in brown rice and tiny little pasta at the end--for a macaw, you could get away with regular non-tiny pasta lol, but I have a dove who can't tear through food so I buy teenie tiny pasta... this is what I buy lol
https://www.target.com/p/signature-stelline-16oz-good-38-gather-8482/-/A-79283467#lnk=sametab
The second option is what I used to do, and it worked very well although it didn't look quite as appetizing after being frozen. I would make bird "smoothies." Same variety of veggies, some unsweetened nut or coconut milk to add liquid to the blender, and then blend. I would mix texture into the smoothie at the end, typically seeds or pellets but the same rice/pasta would work too, but whatever you mix in is going to be exposed to the moisture of the smoothie so keep that in mind.
I would store both options in ice cube trays in the freezer and thaw portions overnight in the fridge. In both cases, what made these presentations work so well is that I mixed my bird's regular diet into them... in the case of the smoothie, I would do it right away and freeze it like that, but there's so much moisture that everything gets kind of weird in the freezer if you go that route. In the case of the food processor, you can do any mixing you want after the portion is thawed and you're getting ready to give it to the bird. I usually top with rolled oats, you can put your seed blend into the bowl and mix with a spoon so that your bird would have to dig through veggies to get to it.
Anyway, whatever route you go down I wouldn't remove your bird's current diet completely as you're right you might end up with a hunger striking bird. I would say remove dry food bowls at night, in the morning give chop with seeds mixed in for an hour or so, and then after your bird has picked through his breakfast you can return the dry food seed mix bowl to the cage. The more you see him accepting the chop, the less you can actually put in the seed bowl every day.