Yeah, turning down the regulator doesn't purge the keg down to that pressure automatically. My method for dropping pressure is to turn off the valve going to the keg, drop the pressure down below my intended new pressure, pull the releif valve on the keg, turn the valve back on, then adjust up to my new intended pressure. Even after that, CO2 will slowly come out of solution as it adjusts to the new head pressure and it will equalize above the set pressure. You have to pull the relief a few times over the course of a couple days (or just continue serving) and the equilibrium pressure will be met.
If you have to keep dropping the pressure for serving, I'd leave the reg where it is and simply shut off the ball valve and pull the relief for a second to drop the keg's pressure. If it drops too much while serving a few pints in a row, just crack the ball valve for a second and shut it again. There's no advantage of having a exactly regulated serving pressure if it's not a balanced system.