it's really really easy on a fretless, you just hit the harmonic then slide up the neck, viola.
on a fretted, it's quite a bit trickier.
it requires swift and delicate shifting up(or down) the neck, with very precise timing.
it's essentially the same as on a fretless, just with more attention to your touch and to not apply too much pressure.
consistently I can slide harmonics about 2 or 3 frets up or down. Once I slid a harmonic up about an octave on a fretted, but I haven't been able to recreate it consistently.
harmonics, require a lot of attention to your touch; attack, pressure, finger placement..etc.
oh yeah, that's doing it with your fingers, Many bassists have Whammy bars on their basses, Vic has one on his classic Fodera, Les Claypool has a couple too.
those enable you to bend harmonics fairly easily.