Its a great help that they will already eat a few pellets and leafy greens.
I donít think that you need to ease them into the new diet, it shouldnít cause any digestive issues, however:
1) If possible, it would be a good idea to weigh your birds as a bench mark measure for any changes. Iíd suggest, first thing in the morning of the second day of the new regime (so you know that they donít have a belly (crop) full of seed) and then keep any future weighing to the same time of day.
2) Try to observe your birds more in this transition time, being vigilant at catching any aggression (brief little squabbles are normal) due to scarcity of their favourite food.
3) Ensure that food (especially seeds) is spread over two+ bowls, and try not to have these side by side, to help prevent one bird hogging them.
Best of luck
Edit - Just adding on here, that different diets can cause slightly different poops. Birds eating a lot of pellets can have poops with the dark portion appearing brown (or the colour of the pellets). Similarly fruit and vegetables can alter colour, and eating a lot of fruit/veg can cause the occasional wetter poop. So, if you notice a change, consider what they have been eating before you panic.