A.Newly replaced strings will go out of tune easily for two reasons. First, when the new string is wrapped around the tuning peg, there's a slight gap between the peg and the string, and a few tunings are required before the string "snugs" itself against the peg. Second, the material that the string is made of is prone to a certain degree of stretching the first few times the string is brought up to tuning tension (this stretching is more pronounced with nylon strings than metal strings).
For these reasons, a single tuning is not enough to stabilize the tuning tension on new strings. For a short time, you will need to continually re-tune the strings until the described conditions stabilize.
(2005/11/22)