In a general sense ATC can change your clearance anytime they need to. Think of a couple of situations (all of these have happened to me or people I know)
1 - You are "Clear to land number 4 behind the crossing G4" - and the timing doesn't work so the G4 is going to be whistling through your runway as you land.
2 - You are "Clear to Land, number one" 30 miles out and in the 10 minutes it takes to get there deer set up camp on the runway
3 - You are on short final and clear to land, the airport has an earthquake
4 - You are "Clear to land behind the departing..." who then aborts the takeoff.
last one - only read about this one - several different times
5 - You are clear to land and someone gets lost and drives onto the runway.
I would expect ATC in everyone of these conditions to tell me to go around (and hopefully to give me a clue as to why)