hi,
Erik, I cannot agree.
AT89C51RD2 manual says:

The 64-K byte Flash is programmed by bytes or by pages of 128 bytes. It is not necessary to erase a byte or a page before programming. The programming of a byte or a
page includes a self erase before programming.

...so it is possible to program any byte without erase full page.

Anyway, it is possible to read page into RAM, modify some bytes, erase page and write its modified content back.

Regards,
Oleg