Execution timewise, P89c51RD2 running in 6 clock mode using 6MHz crystal is equal to AT89c51 running with 12MHz as it by deafault has only 12 clock mode.

So you can load that program into this chip AT89c51.

A note of caution when you are doing it the other way - porting from a AT89c51 to P89c51rd2 : The P89c51rd2 needs the ERAM bit to be enabled if there are any MOVX instructions.

Raghu