hi,
the big difference which may cause a problem is two DPTRs. Well, both devices have them two. But!

AT89S52 controls them via bit DPS (placed in register AUXR1 at 0xA2);
AT89S8252 controls them via bit DPS (placed in register WMCON at 0x96).

So if your code uses both DPTR and switch between them you need to recompile project under new target device (here: AT89S52).

Regards,
Oleg