I always plan to do it more slowly than I do, but, ultimately, uniting our "baby" and "big girl" flocks each year almost always ends up being driven by space concerns.
Namely, we have X amount of space in our backyard, which must accommodate, not only the chickens, but a terrier and four humans, too. Typically, we're staring down spring garden planting season, and I can't envision keeping everyone safe, separated, and sane for even one more day as I work in the garden. (It's a bigger job than you'd think, I tell you!)
This year, I had foresight enough to pop the babies into the big girl coop the night before I was officially at my wits end. The Kid and I went out in the dark with a flashlight and did our transplanting while everyone was asleep. Some people say it's an old wives tale--that if you put chickens together at night when they're asleep, they'll wake up and not know they've ever been separate flocks--but I think it does help.
Another thing that helps is distractions galore. Lettuce, radish tops, strawberries--I tossed all sorts of treats into the chicken run so that when they woke up they'd have much more exciting things to do then peck at each other.
And then, as long as no one's bleeding, I leave them to figure it out. Thus far, the big girls have done plenty of pecking the littles--"the pecking order" is definitely not a wives' tale--but we haven't had bloodshed, which is the goal. For as haphazard as this year's "joining" was, it's gone very well.