I was a Heathrow controller for most of my life and we tried a variety of procedures regarding frequency change. The best was for ATC to instruct the crews to change frequency as soon as they were airborne. I used to do it when there was light under the wheels.
Crews often got mixed up. The best was a EU airline, usually one of the most reliable, who was told "After departure, contact xxx". He changed straight away and called London Control. The controller answered with the usual "squawk ident, climb to altitude 6000 ft". The Controller in the tower was amazed to see the aircraft line up and take-off without a word from him!!!
You can please some of the pilots some of the time....