I had a strong Absinthe juice on one of my REOs. I guess it also depends on whether the two flavors could mix together, such as fruity flavors or sweet flavors. The absinthe was really strong, the odor was really stuck in there, in the cap, O-ring and tubing. I got one of the 6ml bottles, filled it with pure vodka, put it on the REO and let it sit there (upside down so that it reaches the O-ring) for about 5 minutes. Then squonk as much of the vodka out. Rinse the bottle, fill it with warm water, do the same thing as you did with the vodka. Once that's done, if the juice was really strong, the odor might still be there, but you can bet anything that the actual flavor will come out clean. The absinthe is no more. Not a trace of it remains specially after having another juice sitting there for a while.
I just did another swap. I had Mojito juice, which has menthol, and I swapped it for a Chocolate/Mint sweet flavor. Did a quick warm water only rinse, and good as new. There's one little thing I do when I fear the flavor is so damn strong that it will still blend with my new juice. I add a bit of the new juice in a bottle, screw it on the REO and let it soak the tubing and cap/O-ring. After a few minutes, squonk a bit out. Wash/dispose of the bottle. What I hope to do with that is override the odor/flavor with the new one, and minimize even further the possibility of feeling traces of the previous flavor. Then again I only would do that if I had really strong flavors such as cinammon or menthol.