No you won't because the computer takes in what it needs. if it is rated at 30 and you hook up 40 it is only going to take in 30 like light bulbs the house is like at 120w or more (don't know all I know is that it is high) but I have light bulbs that is only rated at 15w and they don't blow up or burn out its just the voltage.
It will not harm your computer to switch the voltage of the power adapters. It is often recommended that the choice of a power adapter should be matched as closely as possible. Find tips on selecting a power adapter at http://www.apogeekits.com/power_adapter_selection.htm.