Hi Filip:
There are two great differences between 8x51 and 8x52. RAM memory, 8x51 has 128 bytes and 8x52 has 256 bytes. Timers, 8x51 has 2 timers (timer 0 and timer 1) and 8x52 has three timers ( timer 0, timer 1 and timer 2). If your software uses the RAM idata zone (0x80h-0xFFh) or the timer2 it wouldn't run properly in your 89C51RB2.