I try to vape it as dry as I can . .with swirling, etc . .and then I put the new flavor in. If it's going to be a bad mix, I'll put just a bit of something neutral and vape that through . .unflavored would be good if I had any.
Like - yesterday I had a tank of mentho/melon candy juice that I thought was awful. I wanted to switch to Halo Tribeca which would be a really bad blend . .so I vaped some vanilla custard through it first. The vanilla custard went OK with the candy melon and blends fine with Tribeca.
That ALMOST worked . .except that nasty menthol/candy melon just won't let go. I'm almost done with the tank of Tribeca and I can STILL taste it a little. For less nasty juices, though, this method works pretty well.