No. Preset memories on the BOSS GX-100 cannot be overwritten. When MEMORY WRITE is executed, the edited settings are saved to a user memory (U01-1 to U50-4).
Note: MEMORY WRITE is required to retain edited settings after powering off the GX-100 or switching between memories. Refer to "Saving a memory" in the GX-100 Reference Manual or "WRITE (Saving to memory)" in the GX-100 Parameter Guide for details.