Can I replace an 8Ohm 1W speaker with a 8Ohn 0.5W speaker?
It will mostly depend on your speaker driver, but likely not. 

Since both speaker has the same 8ohm impedance, the current driven by the driver will be the same on both speaker, and thus will power the same wattage.

Since the original speaker is 1W, if you replace with the 0.5W, the driver will still drive it at 1W, potentially destroying it.

Having said that, you still can replace it, as long as you keep the volume not too high it will work, but bear in mind if you crank up the volume you might destroy your speaker.