So I am designing a switchable voltage 9V/18V power supply and was wondering, what's the best way to switch between outputs, that's the current schematic I came up with:
Original image size
So my question is, will this work and is this the best way or is there a better one?
You should switch the outputs of the regulators. That way you bypass the problem relieved by AaronD and you don't take the risk to have the regulators doing weird things the first ms when they are powered since they are always powered (but I'm not 100% sure about that last part, maybe the regulators are stable even when they have just been powered, but prevention is better than cure...).
You can also switch both input and ouput (using two bridge rectifiers to avoid the transformer tap switching), you've already planned a DPDT relay. You don't get startup instabilities immunity but you get rid of the problem of backpowering the ouputs of the regulators and you don't power the unused regulator.