JAR 25 states that "Except for gear retraction and automatic propeller feathering, the aeroplane configuration may not be changed, and no change in power or thrust that requires action by the pilot may be made, until the aeroplane is 400 ft above the take-off surface."
Reference at
http://www.jaa.nl/jar/jar/jar/jar.25.111.htm
FAR 25 says the same thing.
[This message has been edited by Alex Whittingham (edited 02 May 2001).]