You cant remove existing property, exception to this: can switch skills withing the same skill group, can lower or raise existing properties to their minimum or maximum imbue weight ( like damage increase on weps from exceptional bonus from 40% to 1% or lmc from 8 to 2, etc). Remember though, certain mods can be higher with reforging but not attainable with imbuing, like lmc 10 or stam increase 10, etc