Linear Power Supply Design Tap Switch
Is this any good? 



from a:-



It's circuit diagrams for the Korad (US) /Tenma (UK) series of bench PSUs. Thousands of people have them. They switch the windings as you change output voltage, which you can actually hear quite easily. With two SPDT relays, it might be able to switch in four tap settings. They also automatically switch off the output as you change level. This might be the key to your question.