void setNewStartAddress(Address newStart) {

		message = "";
		newStartAddr = newStart;

		newEndAddr = getEndAddress(newStart);
		if (newStart.equals(block.getStart())) {
			message = "Block is already at " + newStart;
		}
		else if (newEndAddr == null) {
			message = "Start Address is too big";
		}
		listener.stateChanged();
	}
Set the new start address for the block.
Ghidra/Features/Base/src/main/java/ghidra/app/plugin/core/memory/MoveBlockModel.java