"Does the code need to be changed to suite P89C52 MCU"

In your case, obviously it does!

Have you relied upon some undocumented, or undefined feature or behaviour of the Atmel part?
Are you sure that both parts are operating entirely within their specification limits?
Are you sure that absolutely nothing else has changed apart from just the MCU chip? (in a similar post recently, it turned out the there had been a hardware change!)

Have you put the Philips & Atmel datasheets side-by-side and verified that they match?