OK I figured it out. I have to switch the inner piece also, not just the rubber ring.
OK, I'm an idiot. I have a Topeak Turbo Morph G frame pump. I bought it awhile ago and lost the manual.
Now I want to learn how to inflate a Presta tube with this pump before I hit the road, just in case I need pump up a tube. But for the life of me, I can't figure how to switch the head to accept the Presta Valve.
1 road bike (simple, light), 1 TT bike (could be more aero, could be lighter), 1 all-weather commuter and winter bike, 1 Monark 828E ergometer indoor bike
Inside the pump head, there's a rubber thingy that fits presta valves in one direction and schrader valves in the other. Unscrew the lockring, pull the rubber thing out, turn it the other way, and do the same with the tiny plastic bit. Insert and screw lockring back on. You're set!