Thermal Grease? do you mean a thermal adhesive as if it is just grease then it is as the name suggests grease and it doesn't hold it in place only acts as a medium for heat to conduct to the heatsink better with. what type of cpu is is it, is it a socket or slot cpu?
I'm assuming you mean a Heatsink fan combo right? If it was put on with thermal paste then yes, just release whatever locking mechanism is holding the heatsink on and take it off, thermal paste doesn't bond to the cpu, it just enhances the heat transfer between the chip and cooler. Or are you talking about a thermal epoxy cause that's a whole different matter?