Conditions for page write in EEPROM
Typically an EEPROM erases to 1 bits and you can change any 1 to a 0 but you must erase again to go back to 1.  You may need to write the whole page (depending on your interface) but if you write all 0xff you can almost certainly go back and write other values later.  If you write all 0x00 you will have to erase to change the data.  You can overwrite the beginning with its existing data if you have to write in blocks.