Walk through earth cells (use with caution!) makes earth cells behave like water cells, well mostly:
I mentioned in the past that I found the above didn't work properly when Willy attempted to climb up the left-hand wall of 'Halfway up the West Wall' (having walked into that room from 'The Bathroom'). Willy could jump up the wall but got stopped halfway up (appropriately enough for that room!)
Now I have figured out why that happened. The first two POKES NOP out the CP (HL) commands in the checks for Earth blocks in the 'Move Willy (1)' routine. Ordinarily, that means that the subsequent JP Z, #8EBC (jump to the code that deals with Willy hitting his head on an Earth block whilst jumping) doesn't come into effect.
However, if Willy jumps up into the top-left cell of either the upper half or the lower half of the screen, then the L register will hold the value zero (HL holds Willy's attribute coordinates), and so the Zero Flag will have been set by the CALL to #8E9C from #8DF4 (the OR L command at #8EAA is the precise culprit).
As a result, the jump to #8EBC is executed when Willy is located at coordinates (0,0) or (8,0), and Willy is prevented from jumping any higher by the Earth cells.
EDIT: Indeed, I believe (although I haven't tried this out yet) that the above POKES will prevent Willy from jumping straight upwards in the left-most cell-column beyond the top of the screen, or beyond the boundary between the upper and lower halves of the screen, in any circumstances - even if there is no Earth cell present to block him!