The same applies to water. Try showing the bird the water moving around in the container, or get a little drop to drip from the edge near the bird. After a while, the bird usually gets the idea that it contains water. Reduce quantities only once you get a good feel for how much the bird needs. Experiment to make sure you're right.
Birds can get fat, especially on some diets, but err on the cautious side.
Observe the bird carefully for the first few days. A good vet can find some other way to medicate your bird, other than the water. A very small bird has a high metabolism and requires frequent meals, so while one night of hunger may be OK for a healthy bird, more than that can be dangerous. Bird Talk magazine, among other sources, has more information on how to convert birds to new foods. All I want is seed.