1) Don´t know if thats what you asked, but...
@IF
(...)
@THEN
CHANGETERRAIN
terraintype=1
maprect
46,96,46,96,46,96,46,96
@ENDIF
...you only have to insert the ChangeTerrain command, the type of terrain and the coordinates
2) The new terrain (after the ChangeTerrain event took place) is created without any improvements, previous improvements on that square were destroyed by the event. Also units or even complete cities can be wiped out with such events...