Depending on the calf, I've had enough trouble teaching new calves to drink from a bottle - I wouldn't even try teaching a 12 hour old calf to drink from a bucket.
Usually, if I'm planning on moving a calf to a bucket, I'll do it when they're a week or so old, depending on how well they're doing and drinking. For multiple calves bucket feeding is easiest. One or two calves and I prefer the bottle for milk, bucket for grain.
I have a 3 month old calf here that's still on a bottle...LOL. Eats grain, hay, and drinks water from a bucket, but I just never tried giving her milk in a bucket.
Now...I do recall reading something about milk being absorbed better or something like that if the calves are bottle-fed vs. bucket-fed. Can't remember where I saw it, though. Anyone else seen that article?