Anything you do should be gradual--wean them off, don't cut them off. Cockatiels should still have a few healthy seeds mixed in anyway-- just not a lot/not the majority of their food. Think chop/veg, pellets and then seeds (but again, you have to do this very slowly).
Zupreem is an accepted brand but I wouldn't really call it good (I do use it with mine but that is due to the fact that she is very picky and hardly even touches zupreem fruit blend, let alone others). Zupreem has some more natural varieties, which are preferable, but again, if you have to do the fruit blend it isn't the end of the world (just not ideal).
Harrisons and Tops get MUCH higher praise. Roudy Bush is good too, and Zupreem is like... "Fruit Pebbles" compared to "Grape Nuts" or "bran flakes"...and Zupreem's "pure fun-enriching variety" mix for larger birds is pretty carb.. their "sensible seed" mix should be more of a treat than a food (fun, yes..healthy, debatable).
Zupreem pellets better than seeds only by far (although if you research and are diligent, a solid variety of chop can take the place of pellets-but you need to research and weigh etc).
If switching food, be sure to offer their old diet as well and weigh often.
