For Harrisons--they have a transitioning guide on their website. The flavor blend doesn't matter so much, it's whatever your birds will eat assuming they don't have allergies, but you want to start with high potency for 6 months and then switch to maintenance.
The "where to start" page of this PDF goes over that:
https://www.harrisonsbirdfoods.com/wp-content/uploads/2023/07/HANDBOOK-2023-1.pdf
I happened upon this PDF when I was looking for the last one, this guide isn't specific to Harrison's but has a lot of good conversion tips to check out:
https://www.harrisonsbirdfoods.com/wp-content/uploads/2023/07/Quick-Tips.pdf
So to start you would want a high potency fine pellet. This is their "original" flavor in high potency fine:
https://www.harrisonsbirdfoods.com/product/high-potency-fine/
They also have a pepper flavor, which uses I believe cayenne to give it kick:
https://www.harrisonsbirdfoods.com/product/high-potency-pepper-fine/
Lafeber doesn't have the same high potency vs regular blend conundrum. They just offer one single maintenance blend. For a conure, I would recommend either cockatiel sized pellets or their "parrot" sized pellet. My birds are on the cockatiel sized, including a jenday conure and a GCC, but it really depends on the bird whether they like to hold the pellets and bite off tiny pieces or if they like to eat right from the bowl, so you can try both.
Cockatiel:
https://lafeber.com/product/cockatiel-pellets/
Parrot:
https://lafeber.com/product/parrot-pellets/
FYI, Lafeber also sells what they call Nutriberries, which I just call seed balls because that's what they are lol. Also avicakes. They're seeds and nuts that are formed into balls with a binding agent that is fortified with minerals and stuff! My birds have a great time eating them since they have to break it apart to get to their fave seeds. They're a great treat and can replace seed mixes entirely if you desire.
lafeber.com
All of these products are on Amazon, Harrison's I don't think is on Chewy right now but Lafeber is.