Interesting one... I did do conversion from seeds to pellets with all of my birds, some took 3 days, some took 7 years, lol. Also, sorry for the upcoming wall of text.
I know that Roudybush have an apple flavour so I did read from others that they were more successful with roudybush than Harrison's for example. I am guessing they will be more bland compared to your current pellets.
First step would be is to stick to the same sized pellet - if it's for lovebird, if they love millet I would suggest to try with "nibble" size first. They are quite small so maybe the bird will be more drawn to it.
Treat the new pellet as a well... treat. See if the bird accepts from you directly. The bird should first understand that what you are offering is a food, since it already eats pelleted food, it might be possible it will accept it immediately.
My Pascal sometimes has to eat at least a few bites from Harrison's Power Treats because lord, his beak is flaky. The power treats contain lots of fats so it's a good and safe way to offer him something oilier, but he is to my shock not a fan of it. Sometimes he accepts, sometimes not, but if I mix it with his pellets, he will eat it.
So, even if your bird doesn't accept the pellet from you, try putting in another bowl and see if the bird will eat it. You can also mix with the old pellets.
Another way what other people do is in the morning, put for an hour old pellets, then swap with Roudybush. Monitor to see if the bird eats. Never leave the bird without food for more than 6h. Good timing would be for start around 2-3h before putting back the old pellets. If they do not want to eat Roudybush at all, that's ok, you can try introducing it back every couple of weeks, once day they might get interested for it.