Text (SH+errata): Change the target of target spell that targets only a player. The new target must be a player. [Oracle 2009/02/01]
You must pick a new legal target for the spell. [WotC Rules Team 1998/03/01] So if a spell targets "an opponent", you cannot make it target the caster.
You cannot use this on a spell unless the target spell uses the word "target" in its text. If the word "target" is not there, then the spell does not target a player at all. It just affects a player without targeting them. [D'Angelo 1999/06/01]
Note - This card was of type Interrupt. It is now of type Instant. [Oracle 1999/05/01]
