It is never safe to assume that you can just unplug one 8052 variant and simply replace it with another and expect everything to work!!

Do these two parts have 100% compatible pinouts - including electrical & functional specs?

Do these two parts have 100% compatible peripheral sets? (pay particular attention to default states/configurations)

Have you checked that all your SFR settings are compatible? Especially default values?

Does any of your code rely upon the instruction timing of the at89c52?

etc, etc, etc,...