How do I transition a seed-addicted bird to pellets?

Multiple r/parrots threads describe transition periods of 2 months or more, often starting with fruit-flavored pellets like ZuPreem as a gateway before moving to plainer formulations like Harrison's or TOP's. Reviewers caution that some birds reject TOP's outright because it isn't sweetened or dyed, which is a feature for nutrition but a hurdle for picky eaters.