The scripting command AlterCellFlag has the opcode (0x23).
- X-Coordinate (byte)
- Y-Coordinate (byte)
- Byte - New Flag
Sets the cell flag at the specified X/Y coordinate on the current map.
The only known usage of this opcode is in the Cave of Illusion Level 4 where pulling the plug causes all the water tiles (flag 0x0C) to become normal floor (0x00), and Volcano Cave Level 2 where hitting switches causes lava to become safe tiles. Note also, it seems to behave slightly differently on outdoor maps than indoor, but since there are no outdoor maps that call this, it's difficult to experiment, and the point is moot anyway.
TODO: Decide how to handle this opcode on outdoor maps. I've only experimented with Clouds data, so it might be relevant to Darkside. If not, we can either try to work it out by creating our own dummy events, or just make an executive decision, ie, that it can't run on outdoor maps, or that it behaves exactly as indoor maps do.