The erased UFM contains all 0s. Hence, a '0' to '1' transition is possible with a write command whereas going from a '1' to a '0' requires an erase operation. An Erase command erases the sector.
For example:
0xFE -> 1111 1110 to 0xFC -> 1111 1100 (Not possible without erasing UFM)
0xFC -> 1111 1100 to 0xFE -> 1111 1110 (Possible without erasing UFM)